The Subset

Again, easy to understand, from the name.
Sub-set

Something lower, inside the set.

A subset is a set of elements that is inside another set.

Lets say you have a set A that has the numbers from 1 to 5: And that you have another set B, that has the numbers 2 and 3:Of course, it makes perfect sense that this set B is inside the set A. B is a subset of A.
To denote that, you will use this symbol:Indeed, means that "B is a subset of A".

Done?
Next: The Intersection of Sets