Overacting is exactly what the term implies, it's when actors exaggerate immensely and it comes of more ridiculous than dramatic.
Perfect example of terrible overacting:
It's not always bad though, here's an example of overacting that makes the whole thing better:
It often just depends on what they're going for.
That Nick Cage scene isnt overacting, its
bad acting.
Everything he said and did is appropriate for a person being held down as their face is covered in bees against their will. Problem is, he is unconvincing delivering it.
Sometimes full histerics is called for. Yelling, screaming, etc... but if the actor doesn't or cant commit it comes off looking stupid so most actors play it safe and never go all the way there. Its exactly what Ben Steller was saying by "Never go full retard" in Tropic Thunder.
Some of the best acting I have ever seen was by the black chick that won Scream Queens (and was cast in the Saw movies). Because she went full retard cause she didn't know not to cause she had 0 acting experience. So when she would cry on set she would really cry. Snot bubbles and everything. It was like actually happening in front of you.
Then, by the time she did her part in Saw, she had learned to be reserved and her scenes were just average like everyone else.
But if you go back and watch Scream Queens... she was on her way to being GOAT... but she toned it down once she got on.