Limping occurs when a player decides just to call the amount of the posted big blind instead of opening the pot by raising.
It is essential to remember that a player's call is only considered a limp if they are the first to voluntarily enter the pot. If someone has raised before you and you decide to call, that is not considered a limp. Also, if someone already limped before you and you decide to call along, it is usually referred to as over-limping, so the term limp is used for the first player to enter the pot by merely calling.
For example, imagine you are playing a$1/$2 cash game and sitting in the hijack. If everyone folds to you and you decide to call $2, that is limping. On the other hand, if under the gun player already called $2 and you decide to do that as well, that would be considered over-limping.
You should avoid limping for the most part since applying a more aggressive approach and raising or folding is usually a much more profitable strategy for your entire range. That being said, there are some hands that it might be ok to over-limp in passive games, but you should generally never be the first player to limp into the pot.