Rep timing is exactly that, timing of your reps. A rep is a repetition of movement of the weight. For example, on a bench press it would be lowering the weight down, pausing, then pressing the weight back up. How you time reps is very important, much more important than most people think.
Most of the time you’ll see rep timing displayed as 3 numbers, such as 2-1-2. This simply means that the first phase of the movement should take two seconds, the middle phase (or “top”) of the movement should take one second”, and the final phase should take two seconds.
For an example the bench press, as you lower the weight down count “one one thousand, two one thousand” in your head. Once your get to the end the bar should be just above your chest. Hold it there for a count of “one one thousand”, then raise it to a count of “one one thousand, two one thousand”.
Rep timing dictates how long your target muscles will be under stress from the weight. Slowing down the rep timing will mean your muscles are under stress longer, and have to work harder. Slower rep timing also makes it more difficult to cheat and use improper form.
For those looking to build muscle, slower rep timing is generally better. The standard rep timing for bodybuilders is 2-1-2.