This is the number of frames generated per second, ranging from 24 to 60. Higher frame rates theoretically result in smoother animation, but only if the computer can handle the increased CPU and graphics load. On an underpowered machine, increasing the frame rate may have no effect, or may even make the animation less smooth, particularly if expensive effects (e.g. fill, X-ray) are used. Whorld never drops (omits) frames, but it can get behind, in which case the actual frame rate can be considerably lower than the desired rate. If it gets too far behind, it may become unresponsive; see performance.
Note that the frame rate selected here has an important side-effect: it's also used as the frame rate for recorded movies. The default frame rate (25) is ideal in countries that use PAL (e.g. Europe), but may be inconvenient in countries that use NTSC (e.g. USA, Japan). Since NTSC runs at approximately 30 FPS, a movie recorded at 25 FPS, and then output to an NTSC medium (e.g. NTSC DVD) will speed up by 20%. Frame-rate conversion is possible but typically causes unacceptable artifacts. Setting the frame rate to 30 eliminates the speed-up for NTSC, but also requires 20% more CPU and graphics power.
Note that increasing the frame rate also speeds up the animation. To compensate for this, decrease the master speed proportionally.