M6 is a gritty, deadly setting based on pathfinder. The basic concept is that characters are limited to level 6. After level 6 you can still gain feats, at a pre-determined experience level. There are additional ‘capstone’ feats that can be gained, that allow for some of the capstone abilities to be gained.
Mythic abilities can be added as well depending on the campaign.
More can be learned here: M6 external link