Definition of open limp
An open limp occurs when no one has raised yet. Let's say the blinds are 1?/2??? at your table. The player in UTG position folds his hand. You call right after him. Here you will have 3 options: fold, open raise or open limp. We say "open limp" because no one has made a move yet. If you go to open limp and a player after you limps as well, then that player is said to be "limping back", in other words, limping after a limp. A limp is the action of paying the blind only, without raising.
Open limp, a game to be played rarely
Generally, we don't suggest open limp. The reason being that when you raise, you have the initiative and having the initiative will give you an advantage after the flop; that of being able to represent a big hand... Unless it's for limp-reraise, an open limp is generally a game associated with recreational players who play too many hands and want to see as many flops as possible at a low price. When you have a hand to open, especially if you are in early position, the wisdom is to raise preflop and not limp. Also, raising will ensure that you won't be 8 on the flop!