'Limping' or 'limping in' means simply to flat call preflop 1bb rather than raising the BB. It is generally speaking a weak play, often done by beginners. It refers to a preflop call specifically. Silimar plays on later streets are known as 'flatting', as in, a flat call.
Flat betting is a consistent, disciplined strategy that means you are betting the same amount on every game regardless of your confidence level. Flat betting isn't sexy or flashy, but it puts you in the best position to stay in the game long term.