Twelve-bar blues is a song form consisting of three phrases of four bars each. Typically the first two phrases use the same lyrics, but with different harmonies. The third phrase uses different lyrics from the first two, and has higher tension that gets resolved as the next 12-bars begin. In other words, 12-bar blues have a text form of AAB.
The 12-bar blues form has a typical chord progression of I I I I | IV IV I I | V V I I, though musicians tend to modify it for their own music so that it’s not too boring or formulaic. Common modifications include of changing one of the first phrase’s I chord to IV, changing the third phrase’s second V to a IV, and so on. It is also very common to use 7th chords, substitutions, etc.