In music, a mode is an ordered series of musical intervals, which, along with the key or tonic, define the pitches.
A lot of people look at modes as if it were a big mystery, but they are really a fairly simple concept once you learn how to use them on guitar. There are seven tones to the major scale hence there are seven modes to the major scale.
Here are the modes of the major scale.
| I | C ionian | C, D, E, F, G, A, B, C | Cmaj7 |
| II | D dorian | D, E, F, G, A, B, C, D | Dmin7 |
| III | E phrygian | E, F, G, A, B, C, D, E | Emin7 |
| IV | F lydian | F, G, A, B, C, D, E, F | Fmaj7 |
| V | G mixolydian | G, A, B, C, D, E, F, G | G7 |
| VI | A aelian | A, B, C, D, E, F, G, A | Amin7 |
| VII | B locrian | B, C, D, E, F, G, A, B | Bmin7b5 or B half dim |
You simply take a note out of the major scale and play it as if that where the root. So if you start on C and end on C you are in C ionian. Now to find your relative minor simply go down three scale degrees from your root or up five. So the Relative minor of Cmaj is Amin. To find you relative major if you are in minor do the opposite. It also sounds really cool to play the major scale and it's relative minor ate the same time. This is called harmonizing.
D Ionian is the same thing as D major. D Lydian is the same thing as A major but starting, or emphasizing the D. You can start with any note you choose to. But keep in mind that in order to have the feel that your solo is going somewhere along with the chords, the solo should contain elements from the chords. That's where modes come in. If you are in the key of D Major. The Chord A Major can be found in it. Over that chord I would play A Mixolydian or C# Locrian or E Dorian. These are elements from the chord that are going to be emphasized by the mode being played over it. There are other scales that can be played over this, but this should get you started.