According to the Polyamory Society, compersion is "the feeling of taking joy in the joy that others you love share among themselves, especially taking joy in the knowledge that your beloveds are expressing their love for one another." and was coined by the Keristan Commune in San Francisco, which practiced Polyfidelity.

Basically this boils down to being happy when your love(s) is (are) happy. It is the antonym of jealousy.