To call one or more bets with a hand that's strong enough for a raise, with the intention of trapping more money in the pot. Smooth call is like flat call, although it more strongly connotes a powerful hand that one is trying to slowplay.
When a poker player slow-plays a hand he calls a bet with the intention of making a big bet later in the action. He slow-plays to try and induce other players to build up the pot, believing that his hand is so strong it will win out in the end. This type of call would qualify as a smooth call, but a smooth call also describes a difficult call that a player makes, say in the face of a very large bet. A player may also be said to smooth call when he anticipates a raise by a player still to act in the hand.
Usually "smooth call" refers to a situation where you would normally raise because there's a good chance that you have the best hand (or a strong draw), but choose to call instead for whatever reason. So if the situation is a choice between calling and folding it's called a call. If the choice is between calling and raising, it's sometimes referred to as a smooth-call. It's the same as the term, "limping," is actually just calling the big blind, preflop. The term call is still correct in both situations, but the additional terms are more descriptive of the specific situation.