To limp is to open the betting by merely calling the blind in a game that has blinds, or by calling the bring-in for a game that has bring-ins, or by making a minimum-sized bet in other games, instead of raising the pot as would be typical for a player with a strong hand.
Limping is generally applied as a verb only to players in early position or middle position; while technically later players who call are also limping in, they are usually said to be calling the bet instead. This is probably due to the fact that later-position players are usually getting much larger pot odds if a number of early limpers have entered the pot, making it profitable to play a much wider range of hands. Since limping implies weakness, it is not often used to describe these later position players calling a large pot for one bet.
Limping can be a strategic move for players who wish to slow play a strong hand, since limping (by its weak nature) encourages other players to call. It can also work strategically if you have an interesting hand that plays well in a multiway pot, but not in a heads up situation; since limping encourages other callers, it can generate larger pots and deter later position players from raising with hands other than monsters like AA in Hold 'em.
Generally, however, limping has a negative connotation and is often considered to be a weak play.