The theory is that when people are thumbing through books at a book store or library, they often go to a spot around a hundred pages in, and then read an excerpt so see if they like the style and content. This apparently is a natural place for your thumb to stop. Savvy readers know that the first pages and cover blurb might not be the best indication of quality. They're too reworked, too perfect, too aware of the need to hook the reader. By page 99, the writer's true colors will appear.
As Ford Maddox Ford said, "Open the book to page ninety-nine, and the quality of the whole will be revealed to you."
What writers do here is upload their random page 99 (though some cheat, I think, and upload a page they particularly like, or even the first page). Others then rate this on whether they'd turn the page and whether they'd buy the book. You can also write specific comments. This is very helpful feedback to writers. It's also interesting for the rater: you see if your opinions match others', and what particularly struck other people.
If you're a new writer, be prepared for some brutal honesty. It'll help you get the thick skin needed if you ever want to reveal your work to the world at large.
Try it out for yourself. You need to sign up for an account, but it's free.