Old-School Renaissance (OSR) is a corner of the tabletop RPG hobby that goes back to how D&D played in the ’70s and ’80s. Think rules-light systems, characters who die easily, and a focus on clever play over big stat blocks. When the rules don’t cover something, the GM just makes a call. Old-School Essentials, Shadowdark, Cairn, and Mörk Borg are good examples.
Here’s a good primer: https://lithyscaphe.blogspot.com/p/principia-apocrypha.html