If you add too many characters to one line without breaking it with RET, the line will grow to occupy two (or more) lines on the screen, with a curved arrow at the extreme right margin of all but the last of them. The curved arrow says that the following screen line is not really a distinct line in the text, but just the continuation of a line too long to fit the screen. Continuation is also called line wrapping.
Sometimes it is nice to have Emacs insert newlines automatically when a line gets too long. Continuation on the screen does not do that. Use Auto Fill mode (see Filling) if that’s what you want.
Instead of continuation, long lines can be displayed by truncation. This means that all the characters that do not fit in the width of the frame or window do not appear at all. They remain in the buffer, temporarily invisible. Right arrow in the last column (instead of the curved arrow) inform you that truncation is in effect.
Truncation instead of continuation happens whenever horizontal
scrolling is in use, and optionally in all side-by-side windows
(see Windows). You can enable truncation for a particular buffer by
setting the variable
truncate-lines to non-
nil in that
buffer. (See Variables.) Altering the value of
truncate-lines makes it local to the current buffer; until that
time, the default value is in effect. The default is initially
nil. See Locals.
See Display Vars, for additional variables that affect how text is displayed.