Roleplaying games open you up to a seemingly unlimited array of possibilities taking place in many universes. But truly unlimited freedom would literally mean nothing without a context of history, lore and rules for your adventures. Klaatu and mcnalu talk through what open and free mean in roleplaying games with a particular focus on Dungeons & Dragons which Klaatu has been been running for the HPR community in recent months.
If you want to play D&D you can go to the website of its publisher Wizards of the Coast and browse for the player and other manuals. But if you want to play but can't or won't pay then you can use the System Reference Document (SRD) which is published under the Open Game License (OGL). You can even use that document to create and publish your own adventures.
However, if you want to publish an adventure with the full set of D&D rules from official manuals — every monster and every spell — then you need the publisher's approval. To do this for D&D you need to publish through the Dungeon Masters' Guild.