If you compare the A natural minor scale with the C major scale, you will notice that they contain the same notes (i.e., there are no sharps or flats). Because of this, these two scales are referred to as being "relative". The scale of A minor is the relative minor of C major, and the scale of C major is the relative major of A minor.
The C major scale starts on a C note, the A minor scale starts on an A note.
The root note of the relative minor scale is the sixth note of its relative major. E.g., the sixth note of the C major scale is A, therefore the relative minor of C major is A minor.