A Cornelius (Corny) keg is a cylindrical container that was initially used to dispense soft drinks. These days it's cheaper for soft drink manufacturers to sell their fountain drink mix in bags so the Corny keg is now mostly used to dispense beer. It is particularly popular with the home or amateur beer brewer. When you first get a Corny keg it is a good idea to open it up and clean it. This is done via the port at the top of the keg.
Stand the keg on a flat surface so that the end with the circular cap is facing up.
Pull up the pressure-release valve on the top of the cap to release any pressure inside of the keg.
Lift the cap handle. The cap will now be loose in the opening.
Turn the cap itself 90 degrees until it releases from the keg. Remove it and set it aside. You can now empty and/or clean the inside of the keg.