The mode line is a useful summary information about the buffer shown in the window, including its name and current modes. It is located at the bottom of every Emacs Window aside from MiniBuffer windows.
Here is a screenshot that shows a windows with it's mode line (the gray bar at the bottom):
Whether or not the buffer has been modified:
**– modified since last save
--– not modified since last save
%*– read-only, but modified
%%– read-only, not modified
The name of one or more MajorModes or MinorModes of the buffer
Line number of the TextCursor (and Point), and perhaps the column number.
Position in buffer:
Top– the beginning of the buffer is shown in the window
Bot– the end of the buffer is shown in the window
All– all of the buffer is shown in the window
- a percentage – percentage of the buffer above the top of the window.
Coding system of the buffer.
Sources: ModeLine | Emacs Wiki