We won't get too technical here - you can find a lot of technical information everywhere on the Internet - but to make thing simple, as a player, what you need to know is that a game can run with
- a consistent FPS (any number but the FPS doesn't vary)
- low FPS (again, industry standards usually define this as under 30 FPS).
While higher FPS is usually considered better, the most important factor when it comes to performance is arguably consistency: any system running a game at a steady, unchanging 30 FPS will feel smooth, whereas a system where the FPS varies between 60 FPS and 35 FPS will feel choppy. That said, 24 FPS is accepted as the threshold under which the human eye can identify the passage of a frame to the next one.
When talking about performance issues - the game stutters, there are micro-pauses, etc. - people often mean that the FPS are varying wildly, or that they are consistently under 24 FPS.
On PC, the cause of low FPS are often linked to the system requirements for a game not being met, or to graphic settings set at a level that the system can't output fast enough. To explain it roughly:
- If a frame needs to be displayed, but the calculations are still being processed by one or both components, the system will simply stay on the previous frame, and wait for the components to finish to display the next one.
More detailed graphics - or more elaborate physics interactions - require more calculations to be performed, and if the GPU and CPU are not powerful enough to process those calculation in time, a drop of FPS will occur.