Abbreviated as URL, a Uniform Resource Locator is a way of identifying the location of a file on the internet. They're what we use to open not only websites, but also to download images, videos, software programs, and other types of files that are hosted on a server. Opening a local file on your computer is as simple as double-clicking it, but to open files on remote computers, like web servers, we must use URLs so that our web browser knows where to look. For example, opening the HTML file that represents the web page explained below, is done by entering it into the navigation bar at the top of the browser you're using. Uniform Resource Locators are most commonly abbreviated as URLs but they're also called website addresses when they refer to URLs that use the HTTP or HTTPS protocol. URL is usually pronounced with each letter spoken individually (i.e. u - r - l, not earl). It used to be an abbreviation for Universal Resource Locator before being changed to Uniform Resource Locator.
You're probably used to entering in URL, like this one for accessing Google's website:
You can even get super specific and open the direct URL to an image, like this long one that points to Google's logo on Wikipedia's website. If you open that link you can see that it starts with https:// and has a regular looking URL like the examples above, but then has lots of other text and slashes in order to point you to the exact folder and file where the image resides on the website's server.
The same concept applies when you're accessing a router's login page; the router's IP address is used as the URL in order to open the configuration page.
Most of us are familiar with these types of URLs that we use in a web browser like Firefox or Chrome, but those aren't the only instances where you'll need a URL.
In all of these examples, you're using the HTTP protocol to open the website, which is likely the only one most people encounter, but there are other protocols you could use too, like FTP, TELNET, MAILTO, and RDP. A URL can even point to local files you have on a hard drive. Each protocol may have a unique set of syntax rules in order to reach the destination.