What is a mode?
A mode is an “inversion” of a scale:
For instance if you start a scale on the 2nd note of the C major scale you will be playing “D dorian mode”.
THEN WHY DO PEOPLE FIND IT CONFUSING?
That is because of “transposition”.
We can transpose that “Dorian mode of D” to any tonic we desire.
In the example below it is transposed to C as the tonic. How did I do this? Well if the note D [Tonic of D dorian] is two 1/2 steps up from C then what is the note C two 1/2 steps up from? Answer is B flat. So we have a B flat scale staring on the note C.
Below is a Clear explanation and diagram in music notation. Or for my free modes ebook CLICK HERE
For more information please download my free ebook “Modes of the major scale explained in detail” CLICK HEre
Why are the modes useful for the guitar player?
The modes [in this case of the major scale] open up the guitar fingerboard and it becomes easier and easier to connect arpeggios, phrases, quartal, pentatonic and melodic lines together and create smooth transitions across the guitar fingerboard.