Limp
Just calling the big blind preflop instead of raising. Usually weak — it invites the field in cheap.
Say UTG limps in with A-9 offsuit. MP and the cutoff limp behind, the small blind completes, and the big blind checks free. Five players see a 9-6-2 rainbow flop. The limper has top pair, top kicker — and no clean way to play it. Out of position against four opponents, he can't bet for value without folding out everything worse, and he can't check without giving free cards to draws and better nines. Nobody paid a real price to get here, and a hand that would have won the pot as a preflop raise is now guessing against a crowd.
Limping gives up the one sure way to win before the flop: making everyone fold. A raise can take the pot down right there, or trim the field to one or two players who have to put more in with a worse hand. A limp does neither. It invites limpers behind, hands the big blind a free look, and turns a tidy heads-up pot into a multiway mess where one pair is worth less and your bluffs get called by players who paid nothing to stick around.
The most common beginner version is limping a big pair like aces or kings, hoping someone raises so you can reraise and build the pot for them. Usually nobody does, and now the strongest hand at the table is playing a cheap multiway pot instead of a big one it's favored to win. The other version is limping weak, speculative hands up front to see a cheap flop — cheap for you, and just as cheap for the four players now drawing to crack you.