It is how many frames per second your graphic card renders. If you remember the old kids game of drawing stick figures in notepads, and then leafing the pages very quickly to produce a short stick figure amination, then you have the principle of how FPS works. It is not really a movie, just a very fast slideshow, that looks like a movie. Btw, movies in theaters and in then telly work by the same principle. When the FPS goes above 25 or so, the human eye cannot register any isolated pictures any more. Drop the FPS and the movie gets a bit more choppy, drop it to 1-3 fps and it looks like everything is happening at you favourite nightclub.
Normally games have fluid FPS systems, and the processer just works as much as it can. If the processor is strained FPS drops, and sometimes below the 25 FPS area. Things get choppy. If the processor has little to do FPS skyrockets. I can run CS and Q3 with FPS close to 100.
The GPx series has a fluid Processor load and a fixed FPS. The point being that you can lock the amount of Frames Per Second and the game renders from there, never going above or below that number of frames. If you PO goes above 100, your system is working more than it can, so to compensate, GPx slows down time.
In a game where 3/10ths of a second is the difference between a race winning pass and a DNF, slowdown in time is preferable to choppyness. Try and set it to max res and FPS 60 so you have 400-500 PO, and sit down and check passage of time with a stopwatch. You will see that 1 second in the game might be as much as 4 seconds realtime. Many leagues ban this and label it "slowmo driving", as it is easier to do the right thing with 5 seconds to execute the manouver than with 1 second.
As to choppiness, low FPS, this is a matter of taste. As far as I know no league has banned low FPS and as such it is not considered a cheat. As to when the game seems choppy, that is very subjective and partly biological too. Some people go nuts when games go below 30 FPS. I feel that way with 3d-shooters. Others don't seem to notice even very low FPS. If you don't see any choppiness at 13 FPS, lucky you. More bang for the buck, when it comes to PC-systems.
Important note! If you suffer from epilepsy, don't set too low a FPS as this DOES trigger seizures, just like stroboscopic lights and certain japanese cartoons do. Do not fool around with your health.
It's only after we've lost everything, that we are free to do anything.