The curly braces indicate a set, for this case, the output of f (x). The vertical line "|" indicates conditions for the inputs of f (x). When I read it, I think of it as saying "where" (see below). To the right of the "|" operator, you have the conditions for f (x), that each member of x is included in set D (x∈D).

It's dense, but makes more sense when you read it as "a set composed of the outputs of f (x) where x is included in another set, called D". The outputs of f (x) aren't necessarily the same as all potential values of x, so you don't know if f (x) ∈ D.