Simply stated, rep tempo is the time it takes for you to perform a given rep for each exercise. Not many people realize how important this is when it comes to building muscle and strength. And you'll notice very often how gym goers just push around weights without really concentrating on the stimulus they're supposed to be creating with the movement
Every exercise has an eccentric (negative), a concentric (positive) and an isometric contraction
The eccentric is the part of the moment at which you are moving in the direction of gravity. For example: The bench press. When you're lowering down the bar to your chest (moving with gravity) is the negative portion of that exercise.
The concentric portion is working against gravity. In the same example of the bench press it would be the part of the exercise when you push the weight up away from your chest.
The isometric is the one in which the muscle is activated, but instead of being allowed to lengthen or shorten, it is held at a constant length.
For the most part, people tend to focus only on the concentric portion of the movement. And fail to realize that the negative is half of the exercise. And when it comes to building muscle, focusing on both the eccentric and concentric is crucial to activating all those muscle fibers to create the stimulus you're looking for. So its important that you understand the movement as a whole, and not just on moving the weight. Focus on that mind-muscle connection so you can completely feel the movement and demand those muscles to grow.