Gelatin binds +ve charged proteins tannins and other such small suspended particles. It can be used to help clear haze and tannins in beer and wine. It should be used in a beer that has already cleared as much as possible with cold and gravity (time). If you add it to a beer where there is a lot of yeast then the gelatin will bind out to all the yeast and none will be left to bind out your small particles that you wanted to settle (it will overload the gelatin).
Having said that, most beer recipes can be cleared satisfactorily with just some time and cold storage. It will not make much of a change to your taste (unless you are trying to fine out tannins). So if a sparklingly clear beer is not a priority, you don't have to bother.