When talking about the open license of 5th Edition Dungeon’s and Dragons, what people are referring to is the 5e SRD or SRD5. This document outlines all of the content that Wizards of the Coast (WotC) provides free for people to use and grow on. Consider it an open and free rulebook to run D&D 5th Edition.
It is the content contained in here that websites are allowed to post. Anything more, and they have to purchase and use it on D&D Beyond.
This is a link to the document so you can reference it yourself!