A base URL is, basically, the consistent part of your web address.
For example, throughout this training site, you'll note that the address section http://webtech.training.oregonstate.edu always appears in the address bar. This is the base URL. Everything that follows it is known as a URL path.
To find the base URL of your website, go to the site's front page. What you see in the address bar on your site's front page is the base URL of your website.