Flight prices are a bit too steep, don’t you think? You no longer need to regularly check fares on a daily basis on the internet. Google is granting alerts that notify users of any changs in the flight prices on Google Now. Flight information is being tracked based on your recent searches on Google Flights.
The capability to track fares is limited to Google Flights. As far as we know, this feature started rolling out soon and Google expand the tracking to fares from other travel booking websites in the future.
Source: Android Central