E2EE is the abbreviation for End-to-End Encryption. See the complete definition: End-to-End Encryption.
What it means in practice
E2EE is the shorthand for end-to-end encryption. See the full End-to-End Encryption entry for a complete explanation. The short version: the provider cannot read the content. The important caveat: E2EE protects content, not metadata, and not backups stored outside the encrypted system.
Related articles
A journalist was arrested because of an email. — How to communicate with confidential sources safely.