like a part of a unit

A fraction is a part of a whole

The top number tells how many slices you have and the bottom number tells how many slices the pizza was cut into.

We call the top number the Numerator, it is the number of parts you have.

We call the bottom number the Denominator, it is the number of parts the whole is divided into.

NOTE: You just have to remember those names!
(If you forget just think "Down"-ominator)