A Relative minor is built on the sixth note of the major scale. In the key of G the Relative minor would be E minor. In the key of A the Relative minor would be F# minor. The relative minor chord which is the 6th note in the major scale is always the Aeolian mode which is the same in all keys.
In the key of C major the chords can be classified as C = I, Em = III, and Am = VI. The EM and Am can also produce the tonic sound. The Dm = II and the F = IV produce the sub-dominant sound. The G = V, and Bm (flat 5) = VII are the Dominant chords. Play the Bm flat 5 or the G7 and listen how it wants to go back to the C chord both act as dominant chords.
So in laymans terms you can play Am or Em in place of C or vice-versa. Say your playing a song in the key of C and your wanted to solo using the pentatonic scale, which alot of guitarist do, you could play an Am Pentatonic scale (Relative Minor ) or better yet you could also play a Em Pentatonic scale (Tonic Function Chord/Scale). Remember this is a pattern and can be used in all keys. You could think the same way with sub-dominant and dominant chord/scales functions.
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