Modes are a very simple concept but can take some time to really understand and get under your fingers in a way that gets you playing around with them with any authority.

Basically,  a mode is just a scale. When someone mentioned the major scale modes, they are talking about all of the scales contained within the major scale.

For example, you probably already know the C Major scale, which is:

C D E F G A B

If you choose a different starting note (other than the C), and progress through the scale normally from there, you’ll be playing a different “mode”. For example, if you start on D (the 2nd note of the scale), you’d play:

D E F G A B C

That’s known as the Dorian Mode. 

The real key is learning the names of the associated modes and then internalizing the patterns and sounds of those modes. Even though they may have the same notes as their Major parent scale, they will have different sounds and feels to the

 Notice that the 6th mode (Aeolian) is also a regular minor scale.

If you’ve ever heard anyone mention the “relative minor” or “relative Major” scale, this is what they’re talking about. The appropriate minor or Major scale relative to the current mode.

If we are in G Major, for example, the relative minor scale would be E minor (E is the 6th note of the G Major scale. When you play the notes to a G Major scale but start on E, you get an E minor scale).

If we are in A minor, the relative major is? C Major!

So, if we want to take this stuff a step further, we would really learn all of the Major modes as well as all modes relating to melodic minor and harmonic minor as well.

Melodic minor is just like Major, but with a flatted 3rd. So, C melodic minor is:

C D Eb F G A B

It’s a slight difference, but it makes a big impact on the sound of all of the modes.

Harmonic minor is just like regular minor, but with a natural 7th. So, C harmonic minor is:

C D E♭ F G A♭ B

If we take each of those scales and learn all of the different modes associated with each, we get 21 different modes, all with very different sounds (each scale has 7 notes, so there are 7 modes per parent scale. Parent scales being Major, Melodic Minor and Harmonic Minor).

I would suggest tackling these one at a time. First, learn all of your Major modes. Then Melodic minor and finally, Harmonic minor.

That way, when someone says this is a Lydian sound, you will immediately know what that means (Major with a sharp 4! It’s also the 4th mode of Major. C Major and F Lydian have the same notes, but very different sounds when you really emphasize that raised 4th).

I’m big on the shapes of things on my fretboard when learning new harmonic concepts. I would recommend that you write out the name and notes of each Major scale mode and then draw the shape on a fretboard diagram for yourself. I used to simply drill the shapes of each mode until I knew it like the back of my hand.