A Glass Pack is a type of muffler for the exhaust of an internal combustion engine, as used in a car, truck, or motorcycle.
Glass packs essentially consist of a straight through exhaust pipe with holes drilled all over it and another larger pipe packed with fiberglass fitted around it to absorb some of the sound. The advantage of this as opposed to a standard muffler is that it offers significantly less resistance to exhaust gases passing through, thereby increasing engine performance. The main drawback is that it doesn't do nearly as good a job muffling the sound of the exhaust, so they are not legal in many places because they are seen as contributing to noise pollution.