An element of a set is any one of the distinct objects that belong to that set.
In the Featured Image, the set A would contain a number of elements, a, which would share something in common with each other.
A subset of this set of elements may share something in common with some of the elements b from the set of all bs, Set B.
As a whole, Set Theory is often denoted in the language of Logic Notation, but in application, Elements are often arranged in Matrices, and defined by Index Notation.