A 12-bar blues is the most common musical form in blues: a repeating pattern 12 measures (bars) long, almost always played in 4/4 time, and built from three chords on the first (I), fourth (IV), and fifth (V) notes of the scale. The 12 bars split into three four-bar sections: the I chord dominates the first four bars, the IV chord typically appears in the second four, and the V chord arrives in the last four, which resolve back toward I.
In the simplest version the chords run I-I-I-I, IV-IV-I-I, V-V-I-I (in the key of C: C, F, and G). The lyrics usually follow an AAB pattern, where the first line is sung twice and the third line answers it, and a short “turnaround” at the end of the stanza leads into the next repeat (PBS: The Blues; Britannica).
Source: PBS, The Blues Classroom: 12-Bar Blues
Image: Blues guitarist Josh Smith performing, photo by Marc Cooper, CC0, via Wikimedia Commons.