It covers a wide range of disabilities, including vision, hearing, motor, cognitive, and speech disabilities, and spans every kind of digital product: websites, mobile apps, PDFs, software interfaces, and kiosks. Digital accessibility is usually measured against a published standard, most often WCAG, and increasingly required by law rather than treated as optional best practice.
Done well, it also tends to improve usability for everyone, including older users, people on slow connections, and anyone using a device one-handed or in bright sunlight.