Something I get asked a lot and something that is often misunderstood!
The time signature is a really useful little marking that gives you information about the pulse of the music : how it's felt and how it's counted.
The top number tells you how many beats are in the bar
The bottom number tells you what type of beat is one beat (4 means crotchet, 8 means quaver, 16 means semiquaver).
Hope this helps and please let me know if any questions :)