This is an infection/inflammation of the outer ear canal. It is caused by an accumulation of water in the ear canal (usually from swimming) that causes bacteria to grow an infect the ear. Treatment usually involves administering ear drops that contain an antibacterial agent and alcohol (to help dry the ear).
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.