Limping is a poker term that refers to the action of calling pre-flop instead of raising or folding when no one has raised before you.
If the player decides just to call the BB instead of raising it when entering the pot, he is limping. This player is commonly called a limper. Limping is generally seen as a weak move from most positions, and you should avoid this strategy as a rule of thumb.
For example, if you’re playing in a $1/$2 game and everyone folds to you in the hijack position where you decide to call $2, you just limped in. If someone sitting before you limps first by calling and you decide to call as well, that would be considered an over-limp since you are not the first player to limp into the pot.