This is the common term for us uppity women who speak out - against rape, against domestic violence, against injustice. It's perfectly okay for other people to talk about sluts, whores, and other derogatory slurs against women (or other oppressed groups) (with dramatic exclaimation points omg!!!), but if someone speaks up with the weight of their own experience, they get, as Cara from the Curvature did,
"There's no need to act out."
She was "acting out" because she was arguing heavily with someone who was showing heavy disrespect for women and who in the end said "well, what I said wasn't for serious anyway, haha! What're you being so serious for!"
Because, you know, it's not like rape is such a serious issue. It's perfectly okay to mimic people who think rape is hilarious in order to mock them without caring about triggering actual rape victims. And if you DO actually care about such things and speak out against mocking the issue, you're "acting out".
Calm down. It's not that important.