If you are referring to ProtonMail encryption, i think it’s primarily about encryption at rest, so that nobody except you (not even Proton) can read your emails. Unlike other email providers (like Gmail) where the emails could be easily accessed by the email providers whenever they want/need to (like data for ads or legal orders from government etc). While not at rest (i.e., sending/receiving emails), emails between Proton and Gmail are on TLS unless it changed in recent years, unless the email is password protected in which case Gmail cannot read the email until you open it with the password