"Tastes" are essentially how you tell the app what you like, and act as keywords in order to show you places you might like, and the most useful tips for each of those places. The new feature seems to be working quite well, because today Foursquare has announced that in the first 24 hours that the app has been out, users have added over 15 million tastes to their profiles.
Foursquare has about 25,000 taste keywords available to choose from in the app, and says that number is growing every day. And, as happens with features like this, the more people that use it, the better it gets. The recommendation engine already seems to work quite well, so seeing it get better could lead to some impressive results.