What does a genre mean in writing?

A genre is a grouping of works that have some particular feature in common. There is no rigorous criteria for defining a genera; just that 'if you can gather a large enough group of works together that have something in common then you can define a genre by it'.

Vampire fiction is a genre as there is a large volume of works with that element in them.

Newspaper Delivery Boys Fiction is not a genre as there isn't a large number of works centered around that element. Even though Newsies are a far more legitimate (and noble) group than vampires are.