Something is keyed if it has minor modifications to prevent it being inserted in a way that would otherwise fit, but would not work properly.
Floppy disks, for instance are keyed in the front-right corner, preventing them from being inserted more than half-way unless they are the right way up. Inside a PC,
power connectors have two flattened corners prevent them being inserted upside down. American plugs were not initially keyed, but now one pin is wider than the other, ensuring that any fuse is on the
live wire. Newer American plugs have earth wires, removing the ambiguity.