Create a shortcut to your et.exe and amend the target so it looks something like:-
“C:\Program Files\Enemy Territory\et.exe” +set fs_game etpro
That will start ET with ETPro loaded, enabling you to watch demos recorded with ETPro.
FPS = frames per second, and certain values (43,76,125,333) make you jump higher / longer than other values because of the way Quake3 engine games calculate the effect of gravity. The higher of those numbers you can achieve the better, with some jumps only being possible at certain FPS.
What FPS you can get on your PC depends on what processor / graphics card it has, etc., you won’t be able to dramtically increase your FPS from what it is now without a hardware upgrade, although a little tweaking of graphics settings (like setting everything to lowest possible detail) will give you some improvement. The best thing to do is enable the FPS readout (type /cg_drawfps 1 in the console), figure out which of those numbers is the highest you can consistently maintain, and then lock your FPS at that rate with /com_maxfps 43/76/125 etc.