"He ordered the players to wait."
Translation:Li ordonis al la ludantoj atendi.
First of all, ordoni is transitive verb, i.e. it can have a direct object. So you ask for an accusative: Kion li ordonis? This position is taken by an infinitive (atendi). Thus the receiver of the order has to be the indirect object. In Esperanto this is realised with the preposition al, other languages may use the dative case here.
The preposition al can be used with nearly every verb where it does make sense that the action is directed to someone or to something.