Overbet
An overbet is a bet larger than the current size of the pot, used to pile on pressure with a strong hand for value or as a big bluff.
An overbet is simply a bet that's bigger than the pot itself. If the pot has 100 chips in it and you push out 150, you've made an overbet. Standard bets usually land somewhere between a third and a full pot, so anything above 100% of the pot crosses into overbet territory.
It tends to show up later in a hand, on the turn or river, once the board is set and you have a strong read on where you stand. Players reach for it in two opposite spots: to squeeze maximum chips when they hold a monster (a giant value bet), or as a big bluff to make a marginal hand fold under heavy pressure. The oversized number is meant to feel scary and force a tough decision.
As a beginner, you don't need to fire huge bets to play well, but it helps to recognize one when it's aimed at you. A surprise overbet often means your opponent is either very strong or trying hard to look strong, so think about the cards on the board and what they could actually be holding before you call off a big stack.
Example
On a board of K-Q-7-2-9 with 200 chips already in the middle, you hold A-A and shove 300, an overbet of 1.5x the pot, hoping a player with a king pays off your bigger pair.