Per a 9 August 2017 post on Googleblog:
"Updates like expanded text ads have already helped to make mobile ads more useful and informative, and we’re continuing to build on those principles with extensions. That’s why, starting later this month, we’re changing how mobile ads show sitelinks, callouts and structured snippets."
These are links contained in your ads to specific pages of your site. They'll now appear as "tappable" horizontal buttons, like that to "Birthday Flowers & Gifts" in the screenshot below. Google claims this change will cause users to be 2X likely (vs. previous format) to interact with your sitelink.s
Callouts and Snippets:
These are extensions that appear below your ad copy to promote a special offer or something else you want highlighted. Until now these have been displayed below the ad, but now will be in paragraph form inline with the ad. With this change, more callouts and snippets will be eligible to be shown with your ad. Google says users find this layout more engaging.
Apparently as an advertiser you don't need to adjust anything to make these features work - just proceed with campaigns as usual, be aware of them, and look for opportunities they may present to get more bang/buck from your ads.