Definition of smooth call
A smooth call is a call that we do while the action is not finished (there are still players left to talk after us). Let's say for example that a player in position UTG cbet flop 8-9-7. You are on the cutoff and the blinds are still in the hand. You call his cbet while there are still blinds players left to talk after you. You will then be called a smooth call.
Why is it better to be strong when you're a smooth call?
If we go back to our example presented above, the fact of smooth call is "dangerous" if we are not very strong since there is a risk that one of the two blinds will raise, which is not excluded. One of the two blinds can also raise with a draw. The fact of caller without knowing what the other players are going to do will make us lose money in the long run if we are ready to fold in front of a raise. Usually, when you see a great smooth caller, know that he knows exactly what he is doing and he wants to incite the players after him to make a move (either to call with worse or to raise in semi-bluff). A smooth call should be made by a very good hand most of the time.