A cluster set breaks up a larger set into smaller ones and adds a little bit of rest in between.
For the example below, you would complete 7 reps, rest for a short time, do 7 more reps, rest for a short time, then do 7 more reps. This would complete one set. All reps should be done with minimal rest in between.
You may see a bench cluster set programmed as a 3+2+1 with 15 seconds of intra-set rest.
Click on the link here if you want to see a video of Meg giving further explanation of what a cluster set is and why they are programmed: Cluster Sets