This is the 17962nd most frequent Russian word.
фан
Fan (enthusiast or admirer).
In this sentence, 'фан' is used to mean 'fun,' a casual enjoyment or pastime.
Мы занимаемся этим проектом просто для фана.
We are working on this project just for fun.
Here, 'фан' refers to a physical fan, likely an item used for cooling or as part of a collection, depending on the omitted noun context.
Он купил себе новый фан для коллекции.
He bought himself a new fan for his collection.
In this instance, 'фанаты' is derived from 'фан' and means 'fans,' referring to people who admire or support something or someone.