Additional information that can be included in paid search ads. Often used to provide more detail and improve a user’s interaction with an advertisement. It's a Google Ads feature that shows additional information in your top placed ads such as,
location and phone number
site links to pages deeper in your website
call outs
structured snippets
reviews,
address,
pricing,
callouts,
app downloads,
sitelinks,
and click-to-call.
It's Ad extensions help advertisers create richer, more informative ads that take up more on-page real estate, which generally lead to higher Click Through Rates.