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.