F-Droid is like a Linux distribution. And basically, the idea behind that is that open-source means nothing in terms of trustworthiness, unless you compile the source code yourself. The source code is what can be audited, whereas the binaries that the developer provides could be filled with malware and you'd never know about it.

Now, compiling everything yourself is complex and time-consuming, so this is where F-Droid / distros come in. Rather than trusting each and every app developer, you merely need to trust the F-Droid team.