Disjoint in relation to set logic is when you are given two sets that do not share anything in common at all. Here is a simple example. you have a set A that consists of elements 1,2,3,4 and a set B that consists of elements 5,6,7,8. The two sets do not have any of the same elements thus the two sets are disjoint.

Here is also an example of two sets that are not disjoint. you are given set A which consists of 1,2,3,4 and set B which consists of 3,4,5,6. Because the two sets contain some of the same elements the sets are not disjoint.

I found that displaying an example of disjoint sets was the easiest out of all the set logic. Instead of having two circles overlapping, you have two circles separated there for the sets are disjoint (not connected/share similar elements).