Sennin means hermit. Jiraiya is the Toad Sennin, the Toad Hermit.

Orochimaru, Jiraiya, and Tsunade are Sannin's, which means 3 Ninja's.

Sannin isn't a rank, it is just what the villagers of Konoha call the trio.

I can also tell you another fact that makes them special, but am not sure if it was revealed yet in the anime or not, and I don't want you kids crying on me.

Kage is the highest rank you can get. Below that is Jounin.

I think you guys knew that, if not I am sorry, but I think everyone knows that.