Set A is composed of the single member "8"
Set B has the three members "7", "8", and "9"
Notice how "8" is in both sets. We can say that set A is contained set B, or that set A is a subset of set B, because of this fact. Consider a Venn Diagram. Draw a circle representing set B. Fill in the circle with "7", "8", and "9". Then draw a smaller circle just around the "8" to represent this as set A.
Visually you can see that A is inside B completely. There are no portions of A that are outside of B. If we threw a dart and landed in A, then we're also inside B as well. This is the same as saying "we picked out '8' from a hat, so it came from set A or set B".
So to summarize, A is a subset of B. With math notation, we would write this as [tex]A \subset B[/tex]. The "C" looking symbol (it's not really a letter C) means "subset".
