Three lines of four bars. The first line sits on the home chord. The second moves to the subdominant for two bars and comes home for two. The third goes to the dominant for a bar, drops back through the subdominant, and lands home for the last two. In the verse shape blues lyrics often follow, a line is stated, repeated and then answered, and each of the three gets one line of the form, which is where the twelve comes from.
Reading the table
Each row is a bar. Each column after the chord is one note of the blues scale in the key you picked, headed by its degree in that key: the root, the flat third, the fourth, the flat fifth, the fifth and the flat seventh, with the three blue notes marked. A cell gives the note, its class against that bar's chord, and its interval above the chord's root. The chord symbol links to the shape on the guitar.
Where it rubs, and where it doesn't
Read down the flat-third column and the whole character of the form is there. Over the home chord the flat third sits a semitone under the chord's own major third: the rub, and the reason a singer or a guitarist bends that note up towards the major third and so rarely lands on it squarely. When the subdominant arrives, the same note is that chord's flat seventh, a chord tone, and the rub is gone until the form comes home. Over the dominant it rubs again, this time a semitone above the chord's fifth.
The flat fifth rubs in every bar. The fifth of the key never rubs: it is a chord tone over the home chord and the dominant, and over the subdominant it becomes the ninth, a colour note. The subdominant bars are where the scale sits most comfortably, with three chord tones and a colour note; the dominant bar is where the most notes rub. That is a large part of what the form feels like from the inside: a long rub, a release, and one short, hard pull before home.
The last bar
The source prints bar twelve on the home chord, and so does this table. Plenty of bands put something else in that last bar to send the form back round, and the table does not guess which; the quick-change page reads a declared variant of this form rather than an assumed one.