Back in the day when I started web development, websites making their own requests after they loaded wasn't a thing. Eventually, XMLHttpRequest appeared, which let JS do HTTP requests at (page) runtime, and the whole "AJAX movement" kicked off.

Initially, you could literally hit any website with any sort of request, so your website.com could make requests to bank.com, and the browser happily obliged. Of course, this opens up a whole host of issues, so browsers started limiting websites to just being allowed to make requests to the same Origin. But sometimes we want to be able to make requests from pages to other Origins, so CORS (Cross-Origin Resource Sharing) lets you configure your server to tell browsers that "You're allowed to make requests to me, even if you're on a different origin".

This is basically the simplified version of the why and how behind CORS.