Five sit-ups followed by a rest and then five more sit-ups is an example of two sets.
Sets is the number of times that an exercise is repeated for the set number of repetitions. For example if an individual perform 10 squats and rest followed by another 10 squats and rest and then another 10 squats. The person has completed three sets of 10 repetitions. From the question given, five sit-ups followed by a rest and then five more sit-ups is an example of two sets.