The term promotion means different things to different people, but the simple understanding is doing whatever it takes to get your book the attention it deserves!
If someone critiques my book and finds a grammar or spelling error it is my fault. We need to master English to the extent that we can write a good book.
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.