This fraction indicates the number of beats in a measure, and the note value that gets one beat. In 4/4 time, there are four beats in a measure, and a quarter note gets one beat. In 3/4 time, there are three beats in a measure, and a quarter note gets one beat.
The top number of the fraction indicates how many beats are in a measure. The bottom number of the fraction indicates the note value that gets one beat.
If the bottom number is 2, then a half note gets one beat. If the bottom number is 4, then a quarter note gets one beat. If the bottom number is 8, then an eighth note gets one beat.
The music meter can also be used to indicate the accent of the beat. In 4/4 time, the first beat is usually accented, and the third beat is usually a little bit softer. In 3/4 time, the first beat is usually accented, and the second and third beats are usually a little bit softer.