A bass reflex enclosure aims to make use of the radiation from the back of the driver. In a sealed enclosure that radiation is not put to much use, which reduces the efficiency of the system as a whole. One way to do this is to add a ‘port’ to the enclosure. This port is simply a tube (not necessarily round) that acts as a resonator. It is important to note that the port does not allow air from the back of the driver to escape the enclosure. The air inside the tube resonates at a certain frequency (depending on the volume of the tube), much like the noise made when blowing across the mouth of a glass bottle.
This gives a boost at whatever frequency the tube is ‘tuned’ to, which is generally very low to improve bass response.