more is a pager; it does the same thing that type <filename> | more does in MS DOS: it shows you the output of the file for a screen-length, and then gives you the option to view more by pressing the spacebar for another page or "enter" for another line. (The arrow keys, page up, and page down also work as expected. Pressing q for Quit exits the pager without displaying any more of the file.) Older versions of more did not let you go up with the up-arrow key, this was one of the improvements of less -- and was eventually ported to more.
Most commonly used as the target of a redirection; for example cat /etc/fstab | more or ps ax | more.