Encryption means information is transformed in some way such that it cannot be read or changed by unauthorised parties. Typically some kind of secret key is required to read the original information. Modern cryptography uses fancy maths to achieve this.

But "encryption" is kind of an ambiguous thing. Like a lot of services say they use "military-grade encryption!" but the claim is kind of meaningless. What really matters is what data is encrypted, where and by whom.

In a typical computer messaging service, you have the Sender, the Recipient, and in the middle a Server operated by the service provider (eg. WhatsApp/Meta). The Server is needed because directly communicating between two end user devices over the internet is actually pretty hard. The Recipient device may be switched off or out of service range and unable to receive messages, there may be NAT, firewalls or other barriers to establishing connections etc. So the Server handles all messages, temporarily storing messages for retry later, sending out push notifications etc.

In between these 3 parties, you have additional parties involved. The cafe who provides the WiFi; the ISPs who provide the internet connections; other companies or governments who operate the internet infrastructure between ISPs; hackers or rogue employees who gain access to systems and networks; governments who force companies to provide access etc.

So at the very least you want to ensure that the connection between the user (Sender or Recipient) and Server are encrypted to prevent any malicious parties snooping on your messages. A common encryption mechanism uses a pair of keys: a Public key that can be used to encrypt messages, and a Private key that can decrypt them.

End-to-end encryption is a specific type of encryption that takes it a step further; the message content is encrypted on the Sender device (one end), and only decrypted on the Recipient device (the other end). The Server only has enough unencrypted information to route the messages to the correct users/devices, it doesn't need to decrypt the message content. In theory, only the Recipient has the decryption key, so the messaging service provider cannot decrypt it even if they wanted to (or were forced to).

The problem is, end-to-end encryption does not enforce this. You use an app like WhatsApp to generation the keys. There isn't anything that prevents WhatsApp sending a copy of the Private (decryption) key to themselves and reading your messages when they want to. You're trusting them to do what they claim. Then we get to the last part: what is encrypted. It's only the contents of the message. Metadata like how many messages you send, their size, to whom & when, are all accessible to WhatsApp. So end-to-end encryption sounds good in theory, but it you need to understand is limitations.