Vented sub boxes are enclosures for sub woofers with a hole in it to let air escape. This style of box is usually twice the size of a similar sealed box since the airspace inside is much larger.
Vented sub boxes help you tune the output of the sub woofer to a specific frequency range that has been determined by the vent's length, width and diameter. Modifying the port by changing the vent type, such as rounded or slotted vents, can change the frequency of the sub woofer.
Most sub woofers tune to frequencies of about 45 Hz or higher, for maximum output, although this sacrifices a great deal of sound quality. A vented sub box tuned to 33 Hz will help to change the sound quality, allowing the bass to ring clear and the quality to become clearer.