Watch the II7 V I online guitar lesson by Matthieu Brandt from Jump Blues

We've seen that the V - I progression is the strongest chord progression in our music.
And by building on that, we can extend that chord progression by preceding the V chord with IT'S V chord: the V of V. In the original key that comes down to being the IIm7 chord.
To make this progression even stronger we can make that IIm7 chord a dominant 7th chord. The progression would become a II7 - V7 - I .
This can be seen as a short modulation; you're moving to a different key. And this means: move with the chords.
The scale we use for this chord is the mixolydian scale that goes with that dominant 7th chord.