The university is open because it has been declared open in the above-mentioned circumstances. That this is so is not subject to discussion or verification on the part of the addressee, who is immediately placed within the new context created by the utterance. As for the sender, he must be invested 'with the ' authority to make such a statement.
Lyotard explains the language game so simply here. Because the Dean, a person in power and has authority, makes a statement like "the University is open" it is no longer something that can be altered. This is an example that you have to have authority to make a statement that people follow and respect. It also shows that someone with authority can make a statement even if it is not true and people will still believe it as the truth.