Its a port (or vent). When a speaker cone oscillates it creates 2 waves, the front wave which radiates away from the front of the speaker cone and a back wave which obviously comes from the back of the cone.
In a sealed enclosure the back wave is completely independent of the front wave and pressurises the enclosure slightly, this provides a similar effect to pneumatic suspension and can allow the woofer to respond faster and provide a punchier sound.
The ported enclosure has an opening (usually a pipe or a slot) in the enclosure which allows the front and back waves to interact. This lesens the suspension affect which allows the woofer to 'drop' (play lower) further than the sealed enclosure, although with a potential loss in punch. Ported enclosures also have a tuning frequency, this is the frequency which the enclosure performs at its best and produces the highest output.
There's more plenty of info out there on enclosure design and such like but these 2 sites are a good start for the in car applications.