Why can protocols on higher layers be left unchanged?

In his answer to "How does SSL/TLS work?", Luc gives an explanation of how SSL works:

SSL (and its successor, TLS) is a protocol that operates directly on top of TCP (although there are also implementations for datagram based protocols such as UDP). This way, protocols on higher layers (such as HTTP) can be left unchanged while still providing a secure connection. Underneath the SSL layer, HTTP is identical to HTTPS.

In his first sentence, he is saying that protocols on higher layers can be left unchanged.

What does he mean? I know the OSI layers, but I think I've got some knowledge issues here.