This is the 553rd most frequent German word.
selten
Rare or seldom.
Here, 'selten' is an adjective, modifying the noun 'Fund' to describe its uncommon nature.
Das ist ein seltener Fund.
This is a rare find.
Again as an adverb, 'selten' emphasizes the infrequent nature of the occurrences, in this case, of opportunities happening.
Solche Gelegenheiten ergeben sich nur sehr selten.
Such opportunities arise only very rarely.
In this sentence, 'selten' is used as an adverb to describe how frequently an action, 'seeing him,' occurs.