Consider the following sentence:
Léna pense qu'elle en a rencontré un.
I think it's saying that "Lena thinks she's met one". But what's the purpose of "en" here? Why not just say:
Léna pense qu'elle a rencontré un.
In this case, "en" is called a "partitive" because it singles out a part (that is an entity) of a larger set as in "she believes aliens exist and thinks she met one (of them)".