Short answers, pulled from the story.
The System Reference Document is a set of role-playing mechanics available under an open license that Wizards of the Coast released in the year 2000. This document covered the third edition of Dungeons & Dragons and carried an Open Game License attached to it.
Wizards of the Coast chose not to use the Open Game License for the fourth edition which arrived in 2008 but instead adopted a more restrictive Game System License. The resulting System Reference Document contained only lists of concepts and tables from the rulebooks while full texts of the original OGL-licensed rules disappeared entirely.
The latest iteration known as SRD 5.2 arrived on the 22nd of April 2025 following updates to previous versions including an update by May 2016. An updated SRD based on the fifth edition appeared in January 2016 before the full SRD 5.1 was released under a CC-BY-4.0 license in January 2023.
Mutants & Masterminds emerged as one of the earliest games to utilize these open mechanics followed by the Pathfinder Roleplaying Game which became a major competitor within the same market space. These creators relied on the ability to freely produce material compatible with Dungeons & Dragons without paying royalties using the third edition SRD as a basis.
FATE released its own mechanics under distinct OGL-licensed documents for other creators to use while Mongoose Publishing issued editions of RuneQuest that included open licensing options for third parties. Traveller also made its rules available through a System Reference Document format and Zweihänder Grim & Perilous RPG joined this movement with its own set of open materials.