It's a constructor defined with infix syntax. You can make constructors with custom infix names by starting the name with the : character. Alternatively, you could call that constructor App or something and have the same effect:

    data ITerm
      = Ann CTerm Type
      | Bound Int
      | Free Name
      | App ITerm CTerm
      deriving (Show, Eq)

It defines a new infix data constructor. So if `it` and `ct` are values of type `ITerm` and `CTerm` respectively, `it :@: ct` is a value of type `ITerm`. (It could have been written using a prefix operator like `ITerm`'s other data constructors.)