URL
A URL, short for Uniform Resource Locator, is the address of a resource on the web. It tells a browser what protocol to use, what domain to contact, and what path or specific location to request once it gets there.
That makes a URL more than a string you paste into an address bar. It is a compact map of how the web organizes access. https tells the browser which transport convention to use at the application layer. The domain identifies the server namespace being targeted. A subdomain may point to a distinct section of a larger site. The path identifies the resource location inside that site.
URLs matter because they make navigation, linking, and retrieval stable enough for the web to function as a system of connected documents. Hyperlinks work only because the target resource has an address that can be requested directly. Search results also end in URLs. The address bar is therefore one of the most information-dense parts of the browser interface.
They are also useful for diagnosis. If a URL is malformed, points to a deleted path, or targets the wrong domain or subdomain, the failure that follows is not mysterious. It is a bad address, not a failure of the whole web.