A deep link is an ordinary web address that opens your app when it is installed, and the web page when it is not.
On iOS the mechanism is Universal Links, on Android App Links: your domain hosts a small file naming the apps allowed to open its links, and your app declares the domain.
Onelink.to uses deep linking to build open-app-or-download: instead of opening a web page if the app is not installed, a Onelink sends the visitor to download your app.