Definition of overpair
The word overpair could be translated as "overpair". An overpair is a pair higher than the board. If we have a pair of 9s on 4-4-5, we have an overpair on the flop. If the turn is a jack, we have an overpair on the flop, but not on the board (which includes the turn). To have an overpair, we would need to have a pair of queens and better. Most of the time, an overpair will be the best hand on the flop. To use our example above, 99 will have about 65% equity against a classic cutoff range (let's assume we're on the big blind).
Overpair on wet flops vs overpair on dry flops
Let's take a hand like KK preflop. If we are up against an 18% range on a dry flop like 8-8-9, we will have 80% equity against our opponent's full range. But if we change a single card on the flop to make it wet, and an 8 becomes a T which makes a flush draw, the equity of KK drops from 80% to 60% which is considerable. In summary, overpairs should be played slowly on dry boards and quickly on wet boards.