Also known as the "Thank Your URL" the Thank You Page is simply a term for the page your website visitor will land on after filling out a form of some kind. Every form needs some kind of thank you page. It can say whatever you'd like. Sometimes thank you pages are pages that simply say "thank you for filling out the form, we'll be in touch with you soon..."
Here's what my thank you page says after someone fills out my general information request form (catch-all)
Typically the "thank you page" is a page you create on your own site, that isn't on your public navigation bar, that people only see after filling out your form. Think of it as an "unlisted" page. People will only see it after they complete the action you want them to complete (filling out your form)
A thank you page can also be a customize e-promo-kit. This is what many AS members call their "Instant Price Quote" So in this case, the thank you page is the e-promo-kit where we can personalize the experience by merging some of their details like their name.
1Shoppingcart provides you with a very generic general thank you page to use if you haven't created your own. It is best to create your own so you can customize the entire page the way you want it.
You can choose to use your own thank you page or the generic 1Shop thank you page, when generating the form inside 1Shop.