Downloads keywords are the Top 15 keywords bringing the most organic downloads to your app. Right next to each keyword, you get an estimate (in %) of organic downloads for each of them. You can also spy on your competitors’ downloads keywords at anytime!
By adding more keywords to your keywords list, the granularity and precision of this chart will automatically improve as we gain a better understanding of your app position on the App Store. This feature is updated once every 24 hours. In other words, the more keywords you track, the smarter this feature gets!
These estimates are calculated by taking both your app's keyword ranking and the keyword's popularity of the keywords the app is ranking for.
The higher your rank on popular keywords, the more downloads you'll get from that keywords.
Read this blog post for more information on Downloads Keywords.