Library books are cataloged by title, author, publisher, date of publication, and subject heading. A subject heading is an assigned term that tells you something of what the book is about.
For example, you might come up with the keyword "number theory". Relevant books might be cataloged with the subject headings "Number theory", or broken down into "Number theory -- Problems, exercises" or even "Algebraic number theory". You might even, as in the example here, see subject headings in other languages. Knowing these subject headings can make your catalog searches much more effective. But how do you find them?
You can even click these subject headings to find more books cataloged in the same way.