As the name suggests there are 2 pieces of metal (levers) inside the lock. when you insert the key and turn it, the key moves the levers inside the lock, and as the key has different depth of cuts the 2 levers inside the lock will move a slightly different amount. The 2 levers are installed in the lock are slightly different sizes which means that when you turn the key and the levers inside move, they are then in line and the bolt that comes out the side of the lock can slide back and forth ie unlocking and locking the door.
3 lever works on same principle. They would be slightly more difficult to manipulate (pick) as there are 3 levers inside which have to line up to slide the bolt.
Most internal doors seem to be fitted with 2 lever locks such as Basta or Eurospec, and these locks have very few different key variations. E.g. from memory I think Basta 2 levers only have 17 differeing keys.
Hope all that makes sense!