The retailer is the person or company who directly sells your book to the customer; the person who actually buys the book to read themselves or perhaps as a gift.
Some authors think that by selling their book themselves from their own website, they keep the discounts that they would otherwise have to give to retailers.
Unfortunately, without the help of retailers, authors would simply never reach a big enough market to sell their book, even with the best website and Internet marketing expertise.
The mainstream, generic book retailers are so closely aligned with the mainstream, traditional publishers that many authors will not find them to be their most valuable retail channel.
Specialist book shops and retailers that have a specific interest in the subject of a book are a much better option for most authors, because the retailer’s customers are already likely to be interested in your book. All you have to do is let them know about it.