Enter is a command to move from an outdoors room to an indoors room. It is related and functionally opposite to the Leave command.
In Game Description
There are certain areas of the world where it is possible to "Enter" or
"Leave" your current location. You can "enter" an indoors room from an
outdoors room. And you can "leave" an indoors room to an outdoors room.
In an indoors room with multiple exits with only one exit leading to an
outdoors room, simply typing "leave" with no argument will move you to
the outdoors room. If there are multiple exits to outdoors, include a
direction or doorname as the argument.
> leave up
> enter west
> enter rockslab
> leave hiddenhatch