Describe high-level concepts in the specifications page

The specification page mentions some concepts without introducing the concepts themselves.

For example, in the Code Lens Request section:

The code lens request is sent from the client to the server to compute code lenses for a given text document.

As a language server implementor I'm not necessarily familiar with these concepts, so a high-level explanation (or a link to a place where these concepts are described) and an example would help.