SPDT means "Single Pole Double Throw"
That means that there is one set (Single Pole) of contacts that swing each time you throw the switch. A "Double Throw" switch can have two or three positions. The On-On means that this switch has two positions. If you put power into the middle terminal (as Aardil correctly stated) the top terminal will be hot when the switch is down and the bottom terminal will be hot when the switch is up.
If you had a SPDT On-Off-On then your switch would be a three position type where there is a center position with no hot terminal (besides the center).
If you needed another row of contacts you could get a DPDT (Double Pole Double Throw). Every pole adds another isolated circuit path...
And then if you really want to complicate things, you can specify momentary (spring returned) and maintained positions at each throw.
Hope this helps.