Accessible icon design means an icon is never the only way a function is identified: a trash can icon deleting an item needs an accessible name like Delete, either visible or exposed to assistive technology, since icon meaning is not universal and even common icons are misread differently across cultures, ages, and levels of digital familiarity.
Icons also need enough visual weight and contrast against their background to be distinguishable by people with low vision, and interactive icon-only buttons need a large enough touch target, since a small icon with no visible label is difficult for many users to both see and successfully tap or click.