TOC stands for “table of contents”.
It just means “contents”. It's a list telling you what you'll find in a book.
A TOC is sometimes used as promotional tool to whet the appetites of potential buyers of books.
Some smart marketers provide a free download of the TOC and first two three chapters of an ebook. By the time you've read them, you're hooked and want to buy the rest.
If you're selling an ebook, it's a good idea to link items in your TOC to the appropriate chapters in your book. This makes it easy for readers to click on items in which they're interested and find their way around your ebook quickly.
Affiliates can use a TOC, too, to entice potential buyers and get them thinking about the book they'd like to own.
TOC used as a selling tool
See how Ken Evoy cleverly uses several TOCs as a selling tool for his highly aclaimed book Make Your Site Sell! (which is now free).