Accessibility (often abbreviated as a11y) is the practice of designing and developing user interfaces that can be used by people with disabilities. This includes visual, auditory, motor, and cognitive impairments. It ensures equal access and opportunity for all people to interact with information and functionality.
Accessibility is a legal and ethical requirement. Neglecting it excludes a portion of the user base (15% of the world's population) and exposes the organization to compliance lawsuits (ADA, EAA). It also improves usability for all users (the "curb-cut effect").
Accessibility is not a feature; it is a constraint of the medium. We do not "add" accessibility at the end; we bake it into our design system and code from day one. Semantic HTML is the baseline. If it isn't accessible, it isn't finished.