A smooth call is when a player call’s the opponent’s bet, even though the raising in the situation would be the more traditional play.
Players primarily employ this tactic when they want to disguise the strength of their hand. It’s also known as flatting or flat calling.
For example, a player in the cutoff position opens with a preflop raise. You’re on the button, and you choose only to call their bet and not raise it, despite having pocket rockets (a pair of aces). In this case, you’ve made a smooth call.