Outline mode is a major mode derived from Text mode, which is
specialized for editing outlines. It provides commands to navigate
between entries in the outline structure, and commands to make parts
of a buffer temporarily invisible, so that the outline structure may
be more easily viewed. Type M-x outline-mode to switch to
Outline mode. Entering Outline mode runs the hook
text-mode-hook followed by the hook
When you use an Outline mode command to make a line invisible (see Outline Visibility Commands), the line disappears from the screen. An ellipsis (three periods in a row) is displayed at the end of the previous visible line, to indicate the hidden text. Multiple consecutive invisible lines produce just one ellipsis.
Editing commands that operate on lines, such as C-n and C-p, treat the text of the invisible line as part of the previous visible line. Killing the ellipsis at the end of a visible line really kills all the following invisible text associated with the ellipsis.