This is an F-Droid style repository for Android apps, provided by IzzyOnDroid. Applications in this repository are official binaries built by the original application developers, taken from their resp. repositories (mostly Github).
https://apt.izzysoft.de/fdroid/index/info

If you want to make sure it's the right one (and nobody played with DNS). you can use additionally enter this fingerprint in the appropriate field of your F-Droid client: 3BF0D6ABFEAE2F401707B6D966BE743BF0EEE49C2561B9BA39073711F628937A. Or, both combined (as the QR code on the main page does it):

https://apt.izzysoft.de/fdroid/repo?fingerprint=3BF0D6ABFEAE2F401707B6D966BE743BF0EEE49C2561B9BA39073711F628937