Richard Holden, on Google:
Now, we’re going a step further with a new pilot program for price guarantees in the U.S. If you see a flight with the price guarantee badge, it means we’re confident that the price you see today won’t get any lower before takeoff.
I use Google Flights religiously for all trips. At least for my summer trip planning, this hasn’t shown up yet. But I do see myself nudged to use this feature.