A relationship by fate or destiny.
This is a complex concept.
It draws on principles of predetermination in Chinese culture,
which dictate relationships,
encounters and affinities, mostly among lovers and friends.
From what I glean, in common usage yuanfen means the "binding force" that links two people together in any relationship.
But interestingly, “fate” isn’t the same thing as “destiny.” Even if lovers are fated to find each other they may not end up together. The proverb, “have fate without destiny,” describes couples who meet, but who don’t stay together, for whatever reason. It’s interesting, to distinguish in love between the fated and the destined. Romantic comedies, of course, confound the two.