HTTP is a set of rules browsers and websites use to exchange information. It is like a shared language for asking for a page and sending the page's contents back.
When you open a website, the browser makes a request and the server sends a response. The response can include a page, an image, or a message saying that something could not be found.
You may see HTTP in a website address. The secure version, HTTPS, adds protection while information travels between the browser and the site.