First of all, URL refers to the address of a World Wide Web page found in the address bar at the top of the screen. For instance, the URL for Online Sales Pro's Knowledge Base is https://support.onlinesalespro.com/hc/en-us. That is what you would see at the top of your computer screen when you visit that site:
The Destination URL would be where your potential customers come when they click on the link in your ad or emailed message taking them to your site where you tell them all about your product or service. In this case, it's the end destination for your landing page or funnel –– which web page do you want your leads to end up on?