Original: A major and minor scale sharing the same key signature are said to be in a relative relationship. The relative minor of a particular major key, or the relative major of a minor key, is the key which has the same key signature but a different tonic; this is as opposed to parallel minor or major, which shares the same tonic.
Simplified: When a piece of music is in a major key, the relative minor means the minor key which has the same key signature.