Definition of smooth call
A smooth call is a call that you make while the action is not over (there are still players to talk to after you). For example, let's say a player in UTG position bets a flop 8-9-7. You are on the cutoff and the blinds are still in the hand. You call his cbet while the blinds are still talking after you. You are said to be making a smooth call.
Why is it better to be strong when you smooth call?
If we go back to our example presented above, smooth calling 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 may also raise with a draw. Calling without knowing what the other players are going to do will cost us money in the long run if we are willing to fold to a raise. Generally, when you see an excellent player making a smooth call, know that he knows exactly what he is doing and wants to entice the players after him to make a move (either to call with worse or to raise in a semi-bluff). A smooth call should be made by a very good hand most of the time.