on Insecure Writers Support Group:
Keywords are frequently referred to as tags.You may not be aware of this, but search engines don’t care about your book title. It’s true if you enter your book title or your name into a search engine, the results will include your book and your name.
Readers will often search for a book using the name of a best-selling author but readers can’t enter the title or the name of a new self-published author since she and her book have achieved little recognition so far.
Another way readers will search for a book is by using a short descriptive phrase such as ‘fantasy quest’ or ‘regency romance’. These are known as keywords and this is the situation where you want your book to appear in the search results. To accomplish this, it is vital that you develop a set of keywords that will ensure your book title will show up in the reader’s search results.
The keywords you want to use are ones that readers in your genre will use when browsing for a book. These keywords are not necessarily what your book is about: they are the terms a reader will type into a search engine.