Imagine a little harp, with 10 strings, tuned in C major, so you only have C,D,E,F,G,A,B and nothing of Sharps/Flats, now, if you treat the note C as root of your scare, you're playing in C major, let's say you want to change the "happy" feeling for a "Sad" one, you just move your root to A and now you're in A minor... but that's all you can do ? No, if you move the root to the other notes, you get other kind of "feelings" or scales,
C D E F G A B = Ionian (aka "major")
D E F G A B C = Dorian
E F G A B C D = Phrygian
F G A B C D E = Lydian
G A B C D E F = Mixolydian
A B C D E F G = Aeolian (aka "minor")
B C D E F G A = Locrian
That's how you maximize the use of you're little harp for playing different feelings in your "songs".
Now, for a more modern usage, if you copy the scale structure to different roots, let's say Locrian but not beginning in B but in C, you get a scale like this: C Db Eb F Gb Ab Bb and that's still Locrian, is just "C Locrian", in case you don't know how does this actually sounds, or that it may not have a really useful aspect, listen Orff's "Trionfo di Afrodite" which is almost entirely written in this mode.