Baffling or safety foam, It slows fuel cell slosh while providing explosion resistance.
Cell Slosh - Designed for all types of racing including drag racing or circle track racing, safety foam will keep your fuel pump from being starved under hard acceleration or cornering.
Explosion Resistance - If your cell were to become damaged, i.e. in a crash, the foam kind of retains the fuel causing it to drip out of your leaking damaged fuel cell instead of gushing out.
Some even go as far to say that the safety foam keeps weight transfer/shift to a minimum, which provides you with more responsive and predictable handling.
See this link...
http://www.reillyfoam.com/safety.htm