You will sometimes encounter musicians talking about measures or bars in their songs. This is a very simple concept.
Most pop music is in one of two time signatures: 4/4 or 3/4. To keep things simple, think of the time signature as the number you can count off over and over when listening to a song.
4/4: ONE two three four ONE two three four ONE two three four ONE two three four
3/4: ONE two three ONE two three ONE two three ONE two three
Each number counted is a beat.
In 4/4 time, each measure (or bar) has four beats. In 3/4 time, each measure (or bar) has three beats.
In each of the examples above, we’ve counted out four measures.
Sections of music are often divided into eight measures, though you’ll also find sections with four, sixteen, or thirty-two measures.
This doesn’t rule out using any combination you want (including odd numbers), in case you want to change things up.