FPS is known as Frames Per Second in longform. In a mouse sensor, the FPS is the amount of frames the sensor is capturing every second to track distance being travelled. It has a strong relation to IPS below in regards to what it means in terms of performance.
Also referred to as the malfunction speed: IPS, or inches per second, refers to the mouse’s capability to keep up with physically high speed movements. A mouse can only process so much information at a time. When making a large and fast movement with a mouse, the computer may lose some of that tracking information. This means that if you moved 8 inches over really, really quickly, the tracking response or cursor likely won’t move as far as if you moved the same 8 inches slowly.
Note that this works opposite of mouse acceleration. IPS loss is much more common when mouse acceleration is off, though. IPS loss will occur more for low sensitivity users who have to move the mouse over long distances really quickly and the mouse acceleration isn’t on to support that high speed movement.