Used for: Changing the markup of text.
The modify text rule contains a collection of rules that allow you to change the way your text is marked up. You can for example make your entire title/description lowercase or capitalize the first letter of every word. In the screenshot below you can see every rule that falls under the "modify text" rule.
Most of the rules in the dropdown are self-explanatory, they simply do what they say they do. The last two might need a little explanation. "Remove line breaks" deletes little pieces of markup that stand for an "enter" or "newline", which can sometimes make a CSV file a bit messy. The "strip HTML" rule removes all HTML code from for example the description of an item. It's a good idea to always remove the HTML from your descriptions, as most export Channels do not accept HTML code (an exception is Marktplaats).