Every major key has a related minor key and vice versa. Essentially we're talking about two keys that share the same key signature or set of notes. They're relative or related because the two keys share all the same chords and notes. For example: if we're in the key of C, our scale would be C, D, E, F, G, A, B. Notice that there are no sharps or flats. If we were in the key of A minor, our scale would be A, B, C, D, E, F, G. The difference is the tonal center or key of emphasis.
However, with the NNS we almost always think of songs as they would be in a major key. This makes learning chord progressions, scales, and the "whole enchilada" a lot easier to learn.