What is a db9 connector on the back of the computer used for?

A DB-9 connector is a 9-pin connector, approximately 2cm long by 0.75cm wide, with two rows of pins or sockets, a row of 5 and, a row of 4, hence 9 pins total. It is often used in low-speed data communications such as RS-232, and is the connector on personal computers since the IBM PC-AT used for COM ports. There are larger versions with 15 pins, called DB-15 and used for game ports and 10Base5 network AUIs, and 25 pins, called DB-25 and used for earlier COM ports and printer ports among other things.