At about 2:40 of this lesson I explain tritone substitution.

https://www.guitartricks.com/lesson/16068

The tritone part is that the flat 2nd scale degree is a tritone interval from 5th scale degree.  The substitution part is that the flat2 chord is substituted for the the five chord in the progression.  If you expand those chords to dominant 7th chord you find those chord have common tones and you can simply substitute the root notes.

It's more common to jazz because all of the notes of the dominant 7th chord are voiced.  But it's a similar concept in rock and blues, just using simplified voicings.  Basically bII > I is a substitute for V > I.

I explain it in a blues context here.

https://www.guitartricks.com/lesson/13882