"Mortals" is a broad term with some problematic connotations. It's closest analogue is "humanoid" yet that grants a human-centered worldview on a diverse amalgam of creatures. It essentially refers to the most common types of sentient beings on Holos/races that can be played within your average D&D game without too much eyebrow raising—elves, dwarves, humans, halflings, gnomes, dragonborn, half-orcs, half-elves, and tieflings; and the less common supplemental playable races of the aasimar, aarocokra, eladrin, genasi, goliaths, firbolgs, merfolk, and their subraces. Full-blooded orcs are also included because of their connection to the cosmology of Holos. Tritons do not exist in Holos, but the merfolk are considered a mortal race.