URL stands for Uniform Resource Locator and it is what is commonly known as the web address. It is a string of characters which helps you with identifying the web sites and the web pages. The URl is composed of the domain name or the subdomain (which identifies the website) and the page identifier.
To create a custom URL for one of your website pages, click on the Pages tab on the left of the website creation page, select the desired page, then, in the little toolbar at the bottom of the orange panel, click on ☰, then on SEO. You will then be able to custom your page’s URL in the appropriate field.
You can also read our guide: Custom URL: can it improve your SEO?