They're websites that offer discount sales on a variety of things (usually clothes, vacations, home decor, etc). The sales only last a few days (hence the 'flash' part), but the trade off is that sometimes you can find excellent deals, especially on some of the midrange clothing lines. It's super important that you comparison shop the web tho, cause sometimes things are available cheaper elsewhere.
I also use them to find new lines I hadn't heard off, they're also useful for just browsing to make digital look books of things you like
Most of them are 'private'; meaning you have to have a membership, but memberships aren't exclusive, you can just sign up for one at most sites.
The big ones are gilt group, ruelala, myhabit (run by amazon), also ideeli (whose website design I find less aesthetically pleasing and more cataloguey, but they sometimes have good stuff), and hautelook (from whom I have never bought). Jetsetter is fun just to fantasize about dream vacations