What are boundaries and why do I need them?

Any regular visitor to this site knows that I (and some other BWCers) tend to bang on about "boundaries". As in, "you need to set clear boundaries", "this is a chance to make some boundaries", "he's not respecting your boundaries" and so on. And yet, before I found myself spilling my life story to a therapist and learning from her about boundaries, I wouldn't have had a clue what boundaries were. And Lord knows, they weren't part of my life tool kit.
Here's what Oxford has to say about boundaries: 

A limit of something abstract, especially a subject or sphere of activity:a community without class or political boundaries

Here's what Brené Brown says about setting boundaries:

Daring to set boundaries is about having the courage to love ourselves, even when we risk disappointing others. 

From our first definition, we're told that boundaries are abstract. They're the metaphorical line we draw around ourselves to keep ourselves safe. Or rather, they're the metaphorical line we need to draw around ourselves to keep ourselves safe. Most of us don't have that line, or we've been socialized to let people cross it all the time. And rather than enforce it, we swallow our resentment or chastise ourselves for being selfish. Sometimes, we don't even realize it's happening because it's been so long since we had boundaries. But hold on...I'm getting ahead of myself.

What do I mean by "safe"? Boundaries are those lines we create, often subconsciously or by modelling those around us when we're growing up, that allow us to feel safe in this world. They're the lines that, when crossed, make us feel uneasy or threatened. A friend asks to "borrow" our doll but we don't trust her to return it (or we simply don't feel like loaning our doll) so we tell her 'no'. That's a boundary. A parent breaks a promise to take us for ice cream and then tells us to stop being so selfish with our whining because he/she was busy making money to put food on the table. Instead of swallowing our disappointment, we express it in straightforward words. 'I feel disappointed when you don't keep your promises.' That's a boundary. A boyfriend tells us he'd like to take our best friend to a dance because we're out of town. We say 'no, that makes me uncomfortable.' That's a boundary.
Thing is, most of us violate our own boundaries. And, over time, we forget we ever had any.
And so we're still up at midnight baking cookies for a child's class party. We're rescheduling an important meeting because our husband won't stay home with a sick child. We're getting our car back from our teen and the gas tank is empty.
Or...We find out our husband has cheated on us and begging for a second chance. But...he doesn't want to share the details of the affair or the phone passwords because it's a violation of his privacy.
Boundaries.
Our boundaries need to be in place in order to keep us safe.
Our boundaries make it clear that we respect and love ourselves enough to draw clear lines about what we will and will not tolerate in our lives.
Our boundaries make the conditions for reconciliation unequivocal. There's no room for misunderstanding or misinterpretation.
Our boundaries generally include the following:
•There is to be absolutely no contact with affair partner going forward. A letter/text/whatever to that effect is to be sent with the wife copied on it. It needs to state, in no uncertain terms, that the relationship is over, there is regret for having had one in the first place, and that there will be no contact in the future. Full stop.
•If the affair partner tries to re-establish contact, the betraying spouse will immediately tell the betrayed spouse. He will not respond. He will not keep any secrets for the affair partner.
•The betraying partner will provide any/all passwords so that the betrayed partner can check and verify when desired that there is no contact. We know this is hardly perfect (we're not idiots; we know about disposable cell phones, secret e-mails, etc.) but it helps in re-establishing any sort of trust.
•We expect that our questions (asked as respectfully as possible given that we're fighting the urge to bash our husbands over the heads with a shovel) will be answered with full honesty. We have the right to know the full contents of our partner's hearts in order to determine what we're dealing with and how we might respond.
•The betraying partner will be tested to ensure he doesn't have any STDs.
That's the initial list. You can certainly add your own, which might include "no more out-of-town meetings until trust is re-established." Once you get in touch with what you need to feel safe (or safer, as safety will feel relative in the early days post D-Day), you'll be able to establish your own.
As time goes on and we begin to heal from this betrayal, boundaries continue to keep us safe. They allow us to keep that toxic "friend" at arm's length even though she urges more contact. They allow us to say 'no' to commitments that drain us, physically and emotionally. They allow us to reconnect with a basic self-respect that far too many of us have lost. Boundaries are about self-care, not selfishness, and don't let anyone convince you otherwise.
Brown's caveat is crucial to remember: We need to enforce our boundaries even when doing so might disappoint (or frustrate) others. Remember this because if you're not a longtime boundary setter, you are absolutely going to get pushback. You'll get pleading (oh c'mon, we really need you right now. It won't take long...), you'll get anger or aggression (I thought you were my friend but clearly you're not), you'll get sulking (whatever. Do what you want. I don't care). Recognize this as simply the actions of people invested in keeping your boundaries fuzzy, or non-existent. Stick to your boundaries anyway. It will feel sooooooo uncomfortable. You'll feel sweaty. And anxious. Do it anyway. Your head will pound with the sound of your unfamiliar words. Do it anyway. You'll feel terrified that you're going to be left. Do it anyway. People who only stay with you because you make it easy aren't worthy enough for you.
Boundaries aren't about manipulation. They're not about control or getting even or hurting others. They're about NOT hurting yourself. They're about ensuring that all your relationships are free of resentment because you're not doing anything or putting up with anything that makes you resentful.
And if they're not in your toolbox, you need to add them. They're critical to your future happiness.