Preparatory describes something done to get ready for something bigger that will happen later — a preparatory step, a preparatory meeting, preparatory work. It is a formal word you will meet in writing about plans, education, or projects, rather than in casual talk. One important special use: a 'preparatory school' (often shortened to 'prep school') is a private school, in the US usually for younger students preparing for high school, and in the UK often preparing students for entrance to other private schools.