Very simple concept widely used in jazz ( but good for other styles). Like shown in the video, G7 and Db7 are two chords a tritone away (tritone=6 semitones) that share the same 3rd and 7th.
G7= G B D F
Db=Db F Ab B
as you see the notes B and F are common to the two chords. This means that I can also substitute the IIm chord that usually comes before it (as in a II-V-I)
I can play both
Dm7 G7 Cmaj7
Abm7 Db7 Cmaj7
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