This adapter will enable you to use a mouse with a 9 pin D-SUB (Serial) connector to a computer with a 6 pin Mini-DIN (PS/2) connector.
This requires that the mouse handles both protocols. A mouse like this is sometimes referred to as a combo-mouse.
(To the mouse)
(To the computer)
9 PIN D-SUB MALE to the mouse.
6 PIN MINI-DIN MALE to the computer.