Alkalinity is the water's buffering capacity — the carbonate and bicarbonate that resist pH swings and that stony corals use to build their skeletons — targeted at roughly 8–11 dKH on a reef.
Alkalinity is the parameter reef keepers watch most closely because corals consume it daily and it's the first to drift. Keeping it stable matters more than hitting a precise number; swings stress corals more than a steady value slightly off "ideal."