Lightroom is built upon a database. Inside this database, commonly referred to as the catalog, is all the data about your photos. This includes all the metadata created by the camera at the time the photo is taken (shutter speed, f-stop, ISO, etc.) as well as all the data you add in Lightroom (keywords, IPTC data, ratings, etc.) and all the adjustments you make in the Develop module.
Read The Purpose of Importing over at peachpit.com.