Time Signatures tell us what the meter is and what note values comprise the beat. Time signatures are written as two numbers, with one number above the other.
The top number tells us how many beats there are in a measure; this is also the meter. The bottom number tells us what note values comprise the beat.
In the above example the top number is 3: This tells us that there are 3 beats in a measure. The bottom number tells us that the quarter-note gets the beat.