Kalkite is a rare mineral only found so far on planet Ghorman (Andor TV series goes into detail). It is needed to coat the reactor lenses on the Death Star. Some conspiracy Star Wars theorists theorize that Kalkite isn't really required but that be a big plot hole if it was true. Oopsie.
Ok, so I'm going to pretend not to know what Kalkite is, so someone can get to explain it. Ready? "Hey guys. Noob here. What the heck is this Kalkite stuff anyway?"