A time signature indicates the metre of a piece: how beats are grouped into bars. It is written as two stacked numbers. The upper number gives the number of beats in each bar, the lower number the note value that counts as one beat, so 4/4 means four quarter-note beats per bar and 3/4, the waltz metre, three. 4/4 is so widespread that it is called common time and can be written as a large C. Compound metres such as 6/8 group beats in threes.
Almost all jazz lo-fi, like most hip hop and popular music, is in 4/4. Jazz has explored other metres too: the Dave Brubeck Quartet’s “Take Five”, composed by Paul Desmond and recorded in July 1959 for the album Time Out, is in 5/4, split as 3 + 2, and became one of the best-selling jazz singles ever.