You're level 80 now, you've picked up a few sweet upgrades from five-mans and heroics, and you're ready to try a PuG into Naxx. You've found a group that's looking for members, but you're not quite sure how to answer the raid leader's demand: "What's your DPS?" The best way to figure out what your raid DPS, of course, is to measure it while you're raiding – but if you haven't started raiding yet, you can't really do that, can you?
The first thing you'll need in order to figure out what kind of DPS (damage per second) you're capable of putting out is an add-on that keeps track of your performance. Recount is popular and widely acknowledged as a fairly accurate tool. There are plenty of alternatives on all the major add-on sites. Select and install the DPS meter that appeals to you.