Physical-line-mode.el provides a mechanism to move the cursor across "physical lines", which are actual lines displayed on your Emacs screen. Usually Emacs takes a line as "logical" one, which is defined as a string between two newline characters. So if you type C-p (or C-n) on a sentence which stretches for several lines, it seems to skip the part you really want to move to. Physical-line-mode.el makes your Emacs recognize physical lines correctly in the following commands: previous-line, next-line, beginning-of-line, end-of-line.