Watch the Scale Applications 1 online guitar lesson by Robben Ford from Blues Revolution
In this video, Robben explains the concept of a tritone substitution. This means that for a dominant 7 chord we can substitute the 7 chord a tritone away. A tritone is just three whole tones, the exact mid-way point of the chromatic scale. So for a G7 chord, we count up three whole steps to C# (G-A-B-C#) so that C#7 is the tritone substitution for G7, and just like G7 it resolves to C.
Notice that in the G7 chord the B is the 3rd and F is the 7th, while in C#7 the B is the 7th and the F (or E#) is the 3rd - they reverse roles.
We have to alter our scale to play over the tritone substitution. For example, we can play our C major scale over G7 (or you can call it the mixolydian mode, same thing) but for the tritone substitution we need to play C# instead of C, so the scale becomes G - A - B - C# - D - E - F - G just for the C#7 chord.