The amount of frames that are being rendered by your graphics card per second. This can be affected in SL if there are a lot of things being rendered (especially avatars or objects) and your settings are set high. You can view your current FPS via the statistics window (Ctrl-Shift-1 or View -> Statistics Window). If you are experiencing very low FPS you can try lowering your settings (and disabling some options) and/or reducing your Draw Distance, which may help.
A first-person shooter is a type of game where you shoot things while the camera is in a "first-person" view. Half Life 2, Quake, Doom, Unreal Tournament, are all examples of FPS games. Although Second Life does indeed have the facility to act as an FPS, in a rudimentary fashion, it is not intended primarily as a player-v-player combat system. Several groups in-world have, however, created successful combat systems. Second Life also features a default Linden Labs Combat System (LLCS) that can be seen illustrated in a variety of sims. The region known as New Jessie is currently one major example of the idea of first-person shooting in Second Life put into practice. See also Role Playing.