The relative minor of a major scale in a given key is the (natural) minor scale that contains the same note names as that major scale.
Every major scale key has a relative minor.
Given a major scale in a given key, the key / tonic of its relative minor is the 6th note of the major scale.
The key / tonic of a relative major is the 3rd note of a given minor scale.
To see the relative minors of all 12 common major and minor scales on a diagram, have a look at the Circle of fifths.