Can someone help me understand what "acceptance" is?

I’m probably going to mess this up but acceptance as it’s used in its best way in therapeutic/mindfulness contexts is, I think, just seeing reality for what it is right now as clearly as you can..

its not resignation, or approval, or submission, or an admission it will always be the way it is, or that it should be the way it is, but that ‘this is how / what things are right now’.
It’s about not resisting or fighting with reality (at least for a moment). On that tv show, mythbusters, they had a slogan: “I reject your reality, and replace it with my own!’.
Acceptance is the still moment when you stop telling reality what it ought to be and ‘let’ it be what it actually is.

And then you choose what to do next with what’s really there, with what’s available to you.
I think there’s an element of letting go of expectations: I should be this, she should be that, people should be fair, I should be better at life by now…

One of my favourite takes on this is:

’There isn’t a way things should be. There’s just what happens, and what we do.’

And also, yes, my inflexible, mean, critic of a brain can absolutely take all of that (which can be genuinely helpful) and turn it into: ‘too bad, life’s not fair, you can’t change it (because you suck); just endure it and endure it and maintain the status quo and never stop ever’.

The evil twin version can easily be weaponised into maintaining things that harm us - either by ourselves (see above) or by corporate agendas that want us to ‘accept’ and bear the costs of for-profit systems.

But I don’t think it makes pure acceptance bad anymore than water is bad because you can drown in it.