Due to the way we have evolved we have shorter jaw bones but still the same number of teeth. Space is limited at the back of the mouth and as the wisdom teeth are the last to erupt they may become impacted.
The term impacted means that they become jammed in between the second molars and the bone at the back of the jaw and cannot therefore erupt fully. When the tooth is partially erupted a gum flap extends over the crown of the tooth.
Bacteria and food get stuck underneath this flap which makes the area become inflammed and swollen.