When a piece of music is in a major key, the relative minor means the minor key which has the same key signature. It can be found by taking the sixth note of the first scale and playing a minor scale starting on that note. For example: in C major the sixth note is an A. Therefore A minor is the relative minor of C major (C major and A minor share the same key signature: no sharps or flats). C major is called the relative major of A minor.
A complete list of relative minor/major pairs in order of the circle of fifths is: