Frames per second. Frame rate, whatever you might call it, it's how quick your game is proceeding along..processing the information around you.
When you feel like you're stuck in glue and people are blinking by, you might have a crappy frame rate, not just "lag". You might need to reduce your settings on textures and such in the Interface menu to reduce the amount of drag on your Framerate.
There are two ways to view this.
First, mouse-over your Latency bar. (The green gas-gauge thing in the right hand bottom corner on your interface bar.) You should see your Latency (data being transfered and recieved from the game via the internet), your FPS and how much memory your add-ons are taking on.
Secondarily, you can type CTRL+R and the FPS will show in real time your FPS in the middle of your screen at the bottom above your interface bar.
Helps to diagnose problems with "lag".
