Each second the emulated system generates 50-60 images. The individual image is called frame and the number of actually generated frames depends on TV encoding system (PAL:50, NTSC:60). The emulator can display all of the frames, but there are several reasons why it would not to. For example there may be heavy usage of CPU by different process. The emulator would skip some of the frames, i.e. to not display them. Displaying frames is costly process and by skipping some the emulator ensures smoothness of the playback.
Enable Show FPS in the configuration to show how many frames are displayed per second. The text will appear at the top-left corner. This option is available in both global and local settings.
There is availability to explicitely tell how many frames should be skipped at least by the emulator. In both global and local settings you can see a slider called Frameskip. You can choose a value from 0 to 5. The value represents how many frames should be skipped after one displayed (not skipped) frame. But it should be noted that the emulator can skip more than specified.
Tip:
Increase frameskip value to save battery and play longer.