Catalogs are lists of things. Think about a type of catalog that you see almost every day in your mailbox -- the catalog from a retailer like J. Crew or Victoria's Secret. Catalogs like these list the products a business has for sale.
Our library catalog is a list of the contents of our library. In other words, it tells you what the library owns. We call it an "library catalog" because it is in electronic format, and you have to use a computer to access it.