First-Time Downloads is the App Store Connect Analytics metric counting people who download an app to their device for the first time; together with Redownloads, it makes up Total Downloads.
App Store Connect splits downloads in two. A First-Time Download is someone downloading your app to their device for the first time; a Redownload is someone who has downloaded it before adding it again, counted when they tap the redownload button. Total Downloads is the sum; updates and device restores aren't counted. The sales reports keep a related figure, app units, which counts a first Get or Buy per Apple Account, so installs on the same account's other devices aren't counted again. Google Play Console draws a similar line between new users and returning users.
For ASO, first-time downloads are the cleanest read on acquisition. Redownloads measure returning users, and win-back campaigns or reinstalls can move them without any listing change. Apple's own Conversion Rate metric uses total downloads (plus pre-orders) over unique-device impressions, so it includes redownloads too. When testing a new subtitle or screenshot set, compare first-time downloads by source type and territory before and after; a lift concentrated in App Store Search points to the metadata. Paid reports deserve the same split, since Apple Ads reports new downloads and redownloads separately.
Example
A sleep app changes its subtitle and total downloads rise, but most of the gain is redownloads from a win-back email sent the same week. First-time downloads from App Store Search are flat, so the team doesn't credit the subtitle and keeps testing.