"This has been closed."
Translation:Esto se ha cerrado.
Because "it" can't close itself, so you use the "se" as an impersonal pronoun to express that someone somehow closed it. It's like when you have something for sale, you say "se vende" indicating that someone is selling something, but in an impersonal way
Actually, this is an example of the "passive se," which is a little different from the "impersonal se." The latter is used for sentences where the someone is mentioned or implied in an abstract sense, such as, "one can only hope" - sólo se puede esperar.
See here for more info.