The 12-Bar Blues is a progression of chords:
I,I,I,I, IV,IV, I, I, V, IV, I, V. From there any amount of embellishment can be added on.
The 12BB framework is a great way to add a simple constraint to music creation, while still giving you a lot of room to explore ideas like: melody, rhythmic patterns, chord progressions, and scales/soloing. Adding constraints enables deeper exploration of ideas instead of trying to figure out how to start or structure your song.