I've been told that WhatsApp implemented "end-to-end" encryption. In the grand scheme of things, what does this actually mean versus, say, another service which does use HTTPS, such as this website (StackExchange) or some other non-end-to-end encrypted site? Is there some point where even HTTPS/TLS will expose data that doesn't occur in an end-to-end encrypted app like WhatsApp?
End-to-end is where the message is encrypted by the sender and decrypted by the receiver. Nobody in the middle, not the chat provider nor other entities have the ability to decrypt it.
Compare this to a simple chat over HTTPS. Each message is encrypted in transit, just based on the fact that TLS is used. Now, while the intended recipient is another user, the TLS connection is initiated with a server (think Facebook). TLS terminates at the server, and whoever controls the server has the ability to view the messages since they are not encrypted end-to-end. Then, the message may be passed on encrypted over TLS again to the recipient.
The key difference is that the provider is able to view the messages in this case.