There seems to be a command to display only the text in the selected region, leaving the rest of the text hidden. What is the command?
As @Dan commented, you are referring to "narrowing" the buffer. For details read the Narrowing section in the Emacs Manual.
narrow-to-region (bound to
C-x n n by default) will do what you want. Use
C-x n w) to remove the restriction and see the whole buffer again.
In some modes there may be other ways to narrow things. For example, use
narrow-to-defun to focus on the current function in many programming modes; and in org mode you can use