If you use Auto Fill mode, Emacs fills text (breaks it up into lines that fit in a specified width) as you insert it. When you alter existing text it is often no longer be properly filled afterwards and you can use explicit commands for filling.
|• Auto Fill:||Auto Fill mode breaks long lines automatically.|
|• Fill Commands:||Commands to refill paragraphs and center lines.|
|• Fill Prefix:||Filling when every line is indented or in a comment, etc.|