Every page load is made of requests. Learning the basic shape of one helps explain the rest.
A client selects a resource identifier, chooses a method and sends a request. The server returns a status and, when appropriate, a representation such as HTML, JSON or an image.
The same URL can legitimately behave differently for GET and HEAD. A response may also depend on explicit request headers, cache state or content negotiation.
Start by examining the status line, Content-Type, Location and caching headers. A rendered browser page often hides redirect hops and conditional requests.
curl -I https://example.org/
curl -v https://example.org/guide/Read the core concepts, then the request walkthrough. Once responses make sense, move to cache validation and the worked examples.