An accessible description supplements a control’s accessible name with extra context: a password field’s name might be Password while its accessible description explains the exact complexity requirements. Screen readers announce the name first and the description afterward, usually computed from an aria-describedby reference, a title attribute, or adjacent help text.
Getting this right matters most on complex forms, where a label alone cannot carry every instruction a sighted user picks up visually from nearby helper text, character counters, or format hints placed next to the field.