I wish this made more sense to me, can someone help decipher this to someone who lacks context? I'm always working on open source code and Linux but I have to

I wish this made more sense to me, can someone help decipher this to someone who lacks context? I'm always working on open source code and Linux but I have to be honest, I really don't understand the licensing outside of the very basics. My understanding of why this might be bad for open source is that it means, code might end up being littered with GPL and Non-GPL contributions? But would that actually be possible if the project where the contribution is made is GPL? Thanks in advance!