A negative exponent means "per". So 2.0 g cm^{-3} means "2.0 grams per centimeter cubed". g cm^{-3} is just another way to write g/cm^{3}. Notice that 1/x^{y} = x^{-y}, so g cm^{-3} = g/cm^{3}. Similarly, writing
lb gal^{-1} means lb/gal or pounds per gallon.