Frame rate or frames per second refers to the refresh rate of your monitor. While not directly relative to the zombie escape gameplay, it is commonly used and asked after to describe the performance of a particular moment in a map.
If there is a lot of lag on a server or a specific part of the map requires a high amount of hardware performance, the frames on your monitor may lower. By default the refresh rate on monitors should be 60Hz for a perfectly playable gameplay experience, but on big maps (especially ze_maps) this won't be the case, and on tight moments with much network data, fps will lower and performance will be reduced. This leads in many cases to a big dissatisfaction of players, and sometimes quit during the round because they find it unplayable.
When you find that your computer has a hard time rendering the map, you should try lowering your options in the options tab. See this guide to help you improve the performance of your game!