From the White Wolf Wiki, the most complete White Wolf reference

Pathos is raw, undiluted emotion. It is responsible in large part for the existance of wraiths, and is hoarded and treasured beyond almost anything else in the Shadowlands.

Pathos is used for two primary things: to heal the Corpus of a wraith, and to power a wraith's Arcanoi. In order to regain spent Pathos, a wraith can Slumber in their Fetter, use specially endowed Artifacts, make another feel an emotion strongly attached to thier Passions, or, in certain circumstances, drain it from another wraith. Pathos can also be crudely fashioned into a sort of wraithly foodstuff, but this is avaliable only to the most powerful and richest members of the society of the dead and is seen as a sign of pure luxury.

The opposite of Pathos is Angst.