it's defined somewhat arbitrarily but those 7 notes are the notes of the C Major scale. If you just pay them in sequence, that is the scale. Western music is based around 12 tones, but each scale only has 7 of those tones. A piano with black keys allows you to play in any key/scale (based around semitones) since all 12 tones are present but only 7 constitute the major scale.