A library is where books are stored. Prior to the digitalization of documents and books, you can only get books at bookstores or libraries. If you want to get some specialized documents, you will need to travel to the archive (which is a kind of library) that keeps those documents. Modern libraries are normally free to enter, though some libraries may require you to get an access pass or fulfill some other qualifications to enter. You can borrow books from libraries for free in most cases, though some libraries charge a fee to process your membership registration to borrow books.
Just explanating for the millenials who do not know what a library is.