A pause set is a technique that begins to get a little more advanced. In theory, this is a set that utilizes the same weight throughout, but instead of having one failure point, you will extend the set by adding short 15-20 second rests and then performing as many additional reps as you can to then accumulate greater exercise volume. For example, if you were able to do 10 reps of an exercise before you could not lift the weight again, you would take a 15-20 second rest allowing the weight to rest, then perform another set where maybe you only accumulate 4 reps, then rest again, and perform another 2 reps.
By allowing those short intra-set pauses you have now performed 16 reps at the same working weight you were only previously able to perform 10 reps at. This is most used for people trying to build more muscle and strength. Again, this is not something you would use every exercise, or every workout. These advanced variations are meant to be applied strategically to help achieve a training objective.