time signatures determine how many notes are in a bar on the stave. with ought the time signature the performers would be out of time with each outer. the top value how many beats are in each measure in the case of this image there are 4 beats in the bar. the bottom number tells us indicates the note value which represents one beat.
the time signature can also give the feel of the music such as if the song is in 3/4 it gives it a swing feel which is often associated with a waltz.
This is an example of a bar that works with 4/4 with 4 crochets worth 4 beats.