Depending how you use it, too can mean two slightly different things: "excessively" ("Human beings are too weird") or "also" ("I think so too — let's go back to Mars.")
The two versions of too should not be confused with to (or two), but all three words came from the same origin, the Old English word to, which meant "in the direction of." If someone said "I want to go to the joust," she was literally saying, "I want to go in the direction of the joust," and if you said, "I want to go to the joust, too," you were literally saying, "I want to go in the direction of the person going in the direction of the joust." You can begin to see why short words like to and too are handy.