The number on top is the number of notes per measure, and the bottom number is what kind of note.

Let us take for example the most popular time signature, 4/4.
This means there are 4 crotchet notes per measure.

Looking at 4/4, you saw the 4 on top. This tells you that there were 4 of something per measure.

Looking at the bottom number - this can be 1, 2, 4, 8, 16, etc. Look at this chart.

Bottom Number Value
1 Semi Breve
2 Minim
4 Crotchet
8 Quaver
16 Semiquaver
For example:
3/4 = 3 Crotchet notes per measure.
5/2 = 5 Minim notes per measure.
6/8 = 6 Quaver notes per measure.

There are also 2 other common things you might see where the time signature should be.

- Common Time - Same as 4/4 time

- Cut Time - Same as 4/4 but everything is cut in half.
Example: a minim note = 1 crotchet note, a semi breve note = 1 minim note.