The radical master and the kanji master have the same meanings!
This kanji also means main because the master is the main person of a dojo or household or business.
Do you remember back to the radical mnemonic, where the master is floating a water drop above his head? Imagine back to there. Once you do, the most amazing thing happens. The master sends the drop of water into a chute (しゅ) and it goes careening through an intricate series of them.