An app (client side) that allows users to install apps from many repos
A repo system (server side) that allows anyone to host apps in multiple versions
A build system that guarantees that the binary apks are built from the sources
The official f-droid repo hosts only apps built with the f-droid build system. The binaries are signed with the f-droid key and in general are different from the binaries of the same apps that you can find elsewhere, like the GitHub of the apps themseves.
For example IzzyOnDroid repo uses f-droid repo system to host the binaries of many Foss apps as found from each GitHub repo, so not from the f-droid build system.