SEO stands for Search Engine Optimization. Once it was an unknown term; now practically anyone who has a website has probably heard of SEO. But while better known it still remains somewhat unappreciated.
There was a time when an author only had his own imagination, his experience, and, if so blessed, a dictionary as resources. But the world has changed since those olden days.
Every author wants (or should want) to have a tribe. And every time you sell your book that tribe grows. That tribe will include people you don’t know, but it will most likely also include people who you do know.