If you play a C major scale starting on C, it's the C major scale, or C Ionian mode. If you start on the next note, D, and only play the same notes from the C major scale, then that's the D Dorian mode.
If you repeat the exercise all the way through the notes of the C major scale, then you play the following modes, each has its own characteristic sound.
3rd E Phrygian - has a semitone between the first and second degree, sounds kind of Spanish.
4th F Lydian - has a #4th.
5th G Mixolydian - look at it as a major scale with a flat or minor 7th.
6th Aolean or relative minor scale. So in C major it's A minor.
7th Locrian or minor 7b5, sometimes called half diminished.
It's good to play a one octave major scale starting on whatever note you want. Then play all through each mode, you'll get to hear how each sounds.
Playing a G major scale starting on F# you'd be playing F# Locrian mode, so F#, G, A, B, C, D, E, F#. See why it's called a minor 7b5 with A being the minor 3rd, C the b5, E the minor 7 ?