"Frames per second", also known as a "framerate". You kind of mixed the two, heh. Anyway, a frame is a single image that your computer displays. Most monitors can display 60 frames per second, which is also the normal speed that Touhou runs at. If a lot of stuff is happening all at once, or if you have a poor computer, the game might not be able to keep up with the 60 frames per second, so framerate drops. If it says "30fps", that means it's only drawing 30 frames per second instead of 60, so the game is running at half of its normal speed.