Hi 준비하다 means prepare, 준비 중 means preparing. simply when you say verb+중, it means you're in the middle of it or you're on it. it's similar to verb+ing but little bit different. I'm not a teacher so It's hard to explain exactly however..because 준비 is a noun while 준비하다 is a verb, and 중 is also a kind of noun that makes 준비 중 is two words..not a variation of 준비하다. and we usually put a space before a word '중'.