The shutdown command allows you to administratively shutdown an interface. Appending a no in front of this command removes it from the running configuration allowing the interface to be administratively enabled.
shutdown
This image shows you what an interface configuration looks like with the shutdown command
As you can see the interface is administratively shutdown
no shutdown
This image shows you whant an interface configuration looks like when no shutdown command is used, it is simply omitted from the configuration.
As you can see the interface is no longer administratively down, there is no device connected to this port so it is in a down state but it is enabled and able to be used if needed.
