A program is written in source code. However in order for that source code to run on your device, it has to be "complied" into a "binary". Open source apps mean that the source code is avalible, however someone still has to do the compiling. Fdroid does the compiling themselves, which means they can guarantee that the binary in their repository came from the source code, aka it is reproducible from the source code. Izzy gets the developer to compile the code themselves, which means that a malicious developer could submit a binary that's different to the source code.