what is a blow off valve?

is it only for when u have a turbocharger or supercharger?

 

A blow off valve(bov) is only used on forced induction(turbo, s/c) engines. It's not required but it is recomended for many reasons. It releases the pressure build up when you let off the throttle. With the bov, there is less wear and tear on the turbine wheel and it also helps keep the lagtime down between shifts so when you upshift you have instant boost pressure.

You really have to know how a turbo or s/c works to understand this but I hope this helps anyway.
-Paul