Simply put, End to End encrypts the data on the sender's device, and it is decrypted on the receiver's side when it arrives, so it stays encrypted for the whole journey. The only way to decrypt the message is by using the key that only the receiver device has.

Not to be confused with Link Encryption, which works similarly but is able to also encrypt the headers where the routing information is located (IP addresses, MAC addresses, etc). End to End Encryption does not do that; it encrypts the data itself, but not the header.