Your email contains vital information about you, your family, your finances, medical information, etc. This free email service allows you to have encrypted secured email to protect your information.
Proton Mail uses OpenPGP so they can provide end-to-end encrypted messaging with other ProtonMail users and also any PGP user worldwide. This allows them to integrate with other services to create an entire encrypted ecosystem. ProtonMail Free accounts enjoy the same level of security as premium customers.
See this article for a description of what ProtonMail data is encrypted.
Remember that it’s your responsibility to keep your data and your privacy safe.