OPAC means Online Public Access Catalogue. In olden times - well, till the end of the twentieth century - information about the books in a library were written down on cards of a standard size. These cards were then arranged alphabetically in large wooden cabinets. Anyone looking for a particular book had to search by title or author or subject, flipping through numerous cards. Well, this system had its own charm. But, nowadays, library catalogues are stored in databases. and looking for information is a piece of cake. Just type in a few letters and hit Enter. And, voila! Are these the books you are looking for? No? Then search again.
Try the search box in the side panel for this experience.