Literally, 4 running feet.
Poems written in tetrameter have four measures, called feet. In English, the most common foot is called the iamb. (Which is a PAIR of syllables, e.g. beFORE, beLOW, I WANT, I HAVE, etc.)
Iambic tetrameter has four running feet, making a total of eight syllables.
da DUM da DUM da DUM da DUM
