A mode is a scale, basically. The two scales we should already know very well are the Major Scale (Ionian or 1st mode) and the Minor Scale (Aeolian or 6th mode). The difference between modes is NOT the pitches, but the order of them. In the Aeolian mode we begin and end the scale from the 6th position of the Ionian mode.

Above we see A Minor in Orange, and C Major in red. You’ll notice they all have the same natural notes in them, no sharps no flats. They are the same scale, but C Major begins from the 3rd scale position of A Minor, and A Minor begins from the 6th scale position of C Major. Again we notice how through the inversion principle a 6th becomes a relative 3rd. This is the same for each mode. There are 7 modes because there are 7 notes in a diatonic scale. There are 7 notes in a diatonic scale because there are 7 (natural) note spaces available in western sheet music. A -> G, with every other note being one of these notes sharped or flatted. This is convention, but is important because our favorite music is often conventional up to this point.

The modes are:

1Ionian (C from C)
2Dorian (D from C)
3Phrygian (E from C)
4Mixolydian (F from C)
5Lydian (G from C)
6Aeolian (A from C)
7Locrian (B from C)

I’ll include diagrams of the rest of these modes against C Major (C Ionian) here: