Definition of timing tell
The timing tell 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 our eyes, but we can get some information about them. The timing tells are an example of this. A timing tell is information we get about a player or the nature of his hand in relation to the time it takes to play. For example, if I bet and my opponent calls the bet in 2 seconds, I get an information. If he calls in 24 seconds, I get another information. Depending on how long it takes him to make the call, I will get a different view of his hand. That's what you mean by timing tell.
The classic timing tells
Usually, a player who calls very quickly will have a draw. A player who takes his time to call is either very strong and wants to play the one who is unsure of his hand or he is not too strong and hesitates to make the call. A player who takes a lot of 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 lot of time on every move.