Your Voice tab: paste a few samples of your own writing (emails, sales page sections, posts). ADs suggests samples from your site to start with. Press Learn my voice and it writes down how you write: tone, sentence length, words you use and words you never use.
You can read and edit what it learned, keep more than one voice (for example one per brand), and choose which one is active. Every ad, Search ad and keyword plan uses the active voice.
Ads you mark as winners also teach future ads: ADs studies their structure without copying their words.