Marketing is more or less telling people that you’ve written something and maybe they’d like to read it.
At a commercial publisher about half the staff will be in marketing. (Any time you hear someone say that commercial publishers don’t (or no longer) market their books and authors, you can discount anything else that person says … they don’t know what they’re talking about.)
Marketing consists of advertising and promotion. Here’s how to tell the difference: Advertising costs money. Promotion doesn’t. Thus, authors may do promotion but shouldn’t do advertising.
Here’s how to tell if you, as an author, should bother with marketing: If you enjoy it and do it well, you can market your books. If you enjoy it but don’t do it well, you shouldn’t market your books. If you do it well but don’t enjoy it, you shouldn’t market your books. If you don’t enjoy it and don’t do it well, you definitely shouldn’t market your books.
What should you do instead? Write another, better, book and publish it at the best place you can. The number one reason anyone buys a book is that the reader has read and enjoyed another story by that same author. All the other reasons fade into single digit or fractions of percentages.