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. |