I have seen some courses in the university that start with "Topics in this ... Topics in that ...", I have also seen some books that use that word "Topics in Algebraic Graph Theory", "Topics in Structural Graph Theory", so, I've been thinking, what is the difference beetween a book of "Algebraic Graph Theory" and one of "Topics in Algebraic Graph Theory"?, What is the meaning of the word "Topic" in academia context?
"Topics in" usually means that the content is
not meant to be introductory (i.e., the reader is assumed to already have some prior familiarity with the subject), and
not meant to cover "all the important things", but can focus on a selection of (well) topics the author is interested in.
For example, a "Topics in Representation Theory" course can afford to skip both the basic properties of finite group representations (as it is not introductory) and focus (e.g.) on quiver and Hopf algebras. Often, books will have more precise titles than "Topics in...", but lecture notes often take their titles from those of the classes, which are often intentionally vague so they don't have to be changed every year depending on the lecturer. For a department, calling a class "Topics in Algebra" is an easy way to offer a course that will be on something different each year without the trouble of having to rename it every year.