What does it mean to decrease the quote level in the Mail app?

Re: What does it mean to decrease the quote level in the mail app? How would that look?

Seems you could experiment with increasing / decreasing the quote level setting to see what that looks like.

See Apple support's online article: Change Composing settings in Mail on Mac - Apple Support

or

To change these settings in the Mail app on your Mac, choose Mail > Settings, then click Composing.

There is an option to: "increase quote level"

ie: Indent any text you include from the original message when you reply to or forward a message.

If the option is ticked : the quote level is set to increase / therefore unticking will decrease the quote level.

(Image below is from Mac OS Ventura ... so your settings may look different if you have a differeny=t MacOS)