12 Bar Blues Piano Chords

What does chord progression mean? Is the shape of the keys (chords) in the blues., The blues usually only use 3chords, the chords we call them I, IV and V is the basis used for classical, jazz, blues and rock. What is meant by I: this is the basic tone: : Fa and V: sol
Now suppose we do: C C, D, E, F, G, A, B, C mean I: C IV: F and V: G when we play with chords C: base tone.
Blues generally use chord7 and chord9 such as A7, A9, G7, G9, etc. and to make variations in blues can be inserted minor tone but not apart from I, IV and V.Supaya not confused let us learn 12bars blues.
What is meant by bars is not a place for drinking but a fraction of the rhythm being made one. Suppose we play from A7 and we take 4/4 rhythm, meaning 4 taps 1-2-3-4 is name 1bar.How if 2bar? 1 -2-3-4 / 1-2-3-4.
Then what is 12bars? Is in use by blues players from the past until now.

12bars blues is; I-IV / I-I / IV-IV / I-I / V-IV / I-V
With chord A; A7-D7 / A7-A7 / D7-D7 / A7-A7 / E7-D7 / A7-E7
Chord E; E7-A7 / E7-E7 / A7-A7 / E7-E7 / B7-A7 / E7-B7
And can also be tried with other chords by using
Chord7 or chord9