Definition of timing tell
Timing tells exist in live poker, but the term "timing tell" is more associated with online poker. When playing online poker, we can't see our opponents in the whites of their eyes, but we can get some information about them. Timing tells are an example of this. A timing tell is a piece of information you get about a player or the nature of their hand in relation to the time they take to play. For example, if I bet and my opponent calls the bet in 2 seconds, I get information. If he calls in 24 seconds, that gives me another piece of information. Depending on how long it takes him to make the call, I will get a different view of his hand. That's what we mean by timing tell.
Classic timing tells
Generally, a player who calls very quickly will have a draw. A player who takes their time calling is either very strong and wants to play the unsure hand or is not too strong and is reluctant to call. A player who takes a long time to play preflop is either a distracted recreational player who watches TV while playing online poker, or a pro player who plays 22 tables and has so much action that he takes a long time on each move.