Limping means voluntarily entering the pot by calling the big blind instead of raising, when no one has raised yet. The player who does it first open-limps. Anyone who calls behind them overlimps. The result is a limped pot: small, often multiway, and full of hands that did not want to build a pot.
Not every flat call is a limp. The word is for preflop action before a raise.
Limping feels safe because suited connectors, small pairs, and suited aces look better at one big blind than at three. In many home games, it also keeps the peace, and limping aces and kings can hide strength.
A raise still does three things a limp cannot:

Open-limping from early or middle position fails for concrete reasons:
Overlimping in position with a speculative hand is the cleanest exception. Deeper-stack small-blind play sometimes mixes calls. Very deep, passive live games can make suited connectors profitable until someone starts isolating.
Limping a monster to trap works against raisers but backfires against a table of limpers.
Against limpers, isolate larger than a normal open. A common live adjustment is three big blinds plus one per limper. Do not pay them off without a plan.
At low stakes, a limp-reraise is almost always a premium. The default from every seat except the blinds is raise or fold.
Save the pure call for spots where other people have already limped, you have position, and the hand plays well multiway.