An opt-in box is a place on your website where the visitor opts in with their name and email address in order to receive something in return - your free gift.
Opt-in pages (aka landing pages or squeeze pages) are stand-alone pages where you will drive your traffic to, offering them your free gift. They are the most effective way to get visitors to take a specific action - i.e to subscribe to your list.
While it is important to set up both of them, we suggest you focus more on the creation of your opt-in page as it is what converts the most and is a key element of any online business.
You will learn how to create an opt-in box on this video within your membership account:
Blogger module > Creating An Opt-in Box video
And how to create an opt-in page within these videos within your membership account:
M4 Clickfunnels Training V2 > Page Builder videos