Definition of limp
A limp is when you simply pay the big blind. On a 1/2 blind table, if you are in a UTG position, say, you will have 3 options: fold and pay 0, limp (pay the big blind to 2) or raise (to 4 or more). It is possible to limp when no player has raised yet. If a player has already raised and you call, it is called a cold call, not a limp. Generally, it is not advisable to limp since the limping player will not have the initiative and it may encourage other players to limp. Thus, you would be playing a multiway pot with less equity in your hand.
If you do a limp, know why you are doing it
If you know why you are limping, it can be a good game. More and more, world-class players are going to use the limp, especially one-on-one. When you limp, make sure you know why you are limping and that it is the optimal option. For example, when playing 2-player heads-up games, against some very aggressive opponents who 3bet a lot, the limp or limp-reraise option may be a good adjustment to make.