For more information on time signatures in music theory, see: Time signature
A time signature is a musical term defining the number of beats in a single measure of the song (the top number), and which note indicates a single beat (the bottom number). For a time signature of 4/4 (a.k.a. "common time"), 4 1/4 notes (crotchets or quarter notes) fit in one measure, while a time signature of 3/4 (a.k.a. "waltz" or "three-four-time") means that 3 1/4 notes fit in one measure.
The osu! beatmap editor offers several time signatures for mapping, which can be selected from the Timing menu. These include 3/4, 4/4, 5/4, 6/4, and 7/4.