In most cases this is the same as the name of a file you are editing. See Buffers . See Optional Mode Line . At any time, each buffer is in one and only one of the possible major modes. See Major Modes , for details of how the modes differ and how to select one.
Some major modes display additional information after the major mode name.
For example, Rmail buffers display the current message number and the total number of messages. Compilation buffers and Shell buffers display the status of the subprocess. See Minor Modes , for more information. This is not really a minor mode, but is like one. See Narrowing .