Pathology is a Non-Combat Skill that is described in game as "Skill in figuring out how someone or something died. Higher skill allows more details to be determined and more difficult organisms to be examined."
In order to use Pathology the player needs to have some sort of Autopsy Kit in their inventory, like the Basic Autopsy Kit that one can find in the Tutorial Cave or can buy from Mushroom Jack in Serbule.
Once the player has an autopsy kit, all they need to do is examine a corpse and choose the autopsy option that should now be active and hope they don't botch the autopsy.
Autopsies will also give you Anatomy experience related to the type of creature you autopsied, such that if you autopsy a Tiger corpse you would get Pathology experience and Cat Anatomy experience as well.