Short answers, pulled from the story.
A System Reference Document (SRD) is a reference for a role-playing game's mechanics licensed under a public copyright license, allowing other publishers to create material compatible with that game. Dicebreaker described it as a "handy guide on how to use, hack and implement an existing game system for your own purposes".
The first SRD was published in 2000 by Wizards of the Coast, based on the third edition of Dungeons and Dragons, and released under their Open Game License (OGL). It was revised in 2003 following the release of D&D version 3.5.
The D&D 3rd edition SRD formed the basis for independent role-playing games including Mutants and Masterminds and the Pathfinder Roleplaying Game, among others.
In January 2023, Wizards of the Coast announced that SRD 5.1 would be released under the CC-BY-4.0 license. A newer version, SRD 5.2, was released on the 22nd of April 2025.
The 4th edition SRD, released under the more restrictive Game System License in 2008, did not include the full texts of the licensed rules. It presented only lists of concepts and tables from the 4e rulebooks that could be used in a compatible product.
FATE, the Mongoose Publishing editions of RuneQuest and Traveller, and Zweihander Grim and Perilous RPG have each released their own mechanics under OGL-licensed System Reference Documents.