The number of sets is the number of times you did something.
Here are a few examples :
If for example if you walk the dogs twice a day (same walk) you would calculate the value for one walk and then say you did two sets to cover both of them.
If however you go for a 30 min run it's just the one set.
Then if you are doing circuit training you would calculate each item and how long you do them (e.g. cycle 2 mins) and then when you enter into the diary you say how many sets of this you did (e.g. 5x)
The default for sets is 1, so you don't need to touch or alter that field if it does not apply to the type of exercise you are doing.