A PS/2 port is more commonly known as the mouse port. It is a special port that is used to connect a mouse or a keyboard to a computer. This type of port usually works with a mini DIN plug that contains six little pins and it is usually round. You may know these ports by their color, as the mouse port is usually green and the keyboard port is usually purple.
PS/2 ports were created by IBM and most computers come with two of them, usually located side-by-side on the back of your computer's case.