A tetrachord is a building block of modes. It is a succession of 4 notes in a stepwise motion. The distance between the two outer notes is a perfect 4th (2 and 1/2 tones) or an augmented 4th (3 whole tones).
In school we only learn 5 species of tetrachords (Major - C-D-E-F, Minor - C-D-Eb-F, Frygian - C-Db-Eb-F, Lydian - C-D-E-F#, and Harmonic - C-Db-E-F) but in reality there are 12 tetrachords (6 with perfect 4th and 6 with augmented 4th).
Take two Major tetrachords and you get a major scale (C-D-E-F and G-A-B-C). Take two Minor tetrachords and you get a Dorian mode (C-D-Eb-F and G-A-Bb-C) etc.
So out of these 5 tetrachords you can get 3 major and minor scales (natural, harmonic, and melodic) and 7 diatonic modes (Lydian, Ionian, Mixolydian, Dorian, Aeolian, Frygian, and Locrian).
But what is even more interesting, that the ancient Indian system of 72 modes rely on these 12 tetrachords (6 C-F X 6 G-C = 36 modes and 6 C-F# X 6 G-C = 36 modes = total of 72 modes).
In case you want to get to know all tetrachords and experiment with them in your improvisations, here they are:
A good exercise is to take just one of these tetrachords and play something interesting for 10 minutes, exploring different octaves, manuals, stops, tempos, rhythms, meters, textures, forms etc. Notice how each of the tetrachords have a different character and try to reveal it in your improvisations.
It goes without saying that you can build any of these tetrachords from ANY other pitch so color possibilities are virtually endless.