A cluster set is a set that's broken down into several mini sets, with short intra-set rest periods between them. The weight is re-racked or put down during these short rest periods, allowing the muscles to recover before the next mini set. An exercise may have several cluster sets.
Example 1:
10-5-5 20 seconds
This means you complete 10 reps, rest 20 seconds, complete 5 reps, rest 20 seconds, then complete 5 reps
Example 2:
5x4 15 seconds
Here you would complete 5 mini sets of 4 reps with 15 seconds in between