What are projective sets in Set Theory?

Question

I try to write a simply definition of "projective set" in Set Theory context. I read here

1. The operation for generating sets from the open sets is called projection. The projection of $A$ is then a subset of $\mathcal N^n$

2. The class of projective sets is the closure of the open sets under the operations of projection and complementation. The projective sets form a pointclass and are closed (within each $\mathcal N^n$) under unions and intersections (but not countable unions and intersections)

But I don't understand well because projective sets come from Borel sets, Polish spaces context (approach of Moschovakis). Can you help me to clarify?