One of the most important concepts to understand in web development is the relationship between two computers, the client and the server, during a web transaction.
In this relationship, the client is a computer, much like the one you are on right now, that is accessing content on the web throughthe browser or the terminal.
Say we are trying to access a website from the browser on our computer. To do this, we'll type a URL into a browser and hit enter, or click a link; either way, when we do so we'll be sending a request to another computer, the server.
When we send a request to a server, assuming it gets to the server, we will get a message back from that server called a response. This response will be accompanied by a status code, letting us know if we were successful, and if so what happened, or alternatively, if something went wrong, and if so, where it went wrong.
All the work on the web is accomplished by the request-response cycle.