Kalkwasser is calcium hydroxide (aka lime water).
To explain it simply, it is a supplement that is used by some people to replenish the calcium levels in their tank that can be depleted by its inhabitants (clams, SPS, etc). It's also used as a paste to kill aiptasia and majano nuisance anemones.
My suggestion, if you're new to the hobby, stay far, far, far away from Kalkwasser.