Domain Forwarding is a service you can ask from your domain registrar (where you bought your domain name) which allows you to "forward" the domain to another site. What happens to the forwarded domain name is that when a user types your domain name into a browser, the site that comes up is the site you have forwarded that to. You can find more about domain name forwarding via https://razorhosting.net/domain-fowarding
This can be useful when you have several variations of a website or company name, or some common misspellings of that name, but you want them all to go to the same place. Let's say your company name is Tammi's tool. Tammistools.com you have a domain name, but you know that many people will automatically type in Tammy instead of Tammi, so you also purchase tammystools.com, then forward that to the correct spelling so, even if people put it in incorrectly, they still get to you.
When you simply "forward" domain name, the ultimate goal will appear in the address bar, so that even if someone types tammystools.com, it will end up saying tammistools.com. There is another level of forwarding, however, that will remain with the original spelling instead. This is called forwarding with masking.
When you also "mask" the domain name, what is originally typed will always stay in the address bar as long as the person is on your site. There are many reasons you may want to do this, but the most common is with affiliate links.