I also agree with Paul’s description. Pre-exhausting is an effective superset where you take two exercises and combine them for the same muscle group. The idea is to isolate and then perform a compound movement. This technique and the idea behind it is to take the muscle beyond the normal point of exhaustion. It helps achieve muscle growth and muscle fiber stimulation–beyond that of doing straight up sets and reps.
It is a great technique and one I would consider as an advanced strengthening technique, so use caution if you are using this with a client. I would make sure they are conditioned first with regular sets of strength training. When using the pre-exhaust technique, they should be doing less weight than normal on the compound/second exercise. It also, of course, depends on the goals of your client.