During the promotion, you can order any size Frappuccino, customizable to your liking, at half the normal price. Given that Frappuccinos start at around $4 and go up to $6 plus, it'll end up costing you $2 to $3 (more, if you go crazy with the syrups and sauces) per Frappuccino.
If you're looking to try something new off the menu, like maybe an Orange Creme Frappuccino or a Snickerdoodles Frappuccino, here's a handy list for reference. If you're wondering what your customization options are, I've got you covered there as well.
If you happen to be a Starbucks Reward member, you get an extra hour (to 6 pm) to the deal period on May 6 through May 10, 2014.