A mouse. This has a small D-type connector on the end (not like PC mice) , and plugs into the port on the far left, labelled "1 mouse"
Note that this is NOT an old PC serial mouse. You need an Amiga-specific type, or you could invest in a PS/2 -> Amiga mouse adaptor.
And, as others have said, NEVER plug/unplug anything while the Amiga is on! The Amiga is very fragile in this sense.
