[JepZ]

I am neither a ProtonMail nor a Gmail user so this might be completely wrong, but as far as I understand it, the short answer to your question is:

ProtonMail offers an easy access to a client which supports End-To-End encryption for your emails.

So nobody besides the sender and the receiver can read the content of the email. Traditional emails are more or less plain text files which (usually) get encrypted for transfers between mail servers, but every mail server involved in the transmission can read the content.