Written as two stacked numbers, a time signature tells musicians how to count. 4/4 has four quarter-note beats per bar and covers most pop, rock and electronic music. 3/4 has three beats and gives the lilt of a waltz. 6/8 groups six eighth notes into two pulses of three, the swaying feel of many ballads and Irish jigs.
Unusual signatures such as 5/4 or 7/8 create a sense of asymmetry. They appear in progressive rock, jazz and many folk traditions, including Balkan music.
Using it in a prompt
Mention the time signature for a specific feel: "in 3/4 like a waltz" or "in 6/8 with a rolling feel". If you leave it out, most generators assume 4/4.