Accessibility (a11y) means that websites are designed and developed so that everybody can use them, regardless of visual, auditory, speech, motor or cognitive impairments.
Accessibility is not limited to providing a usable website for impaired people; an accessible website also benefits people without impairments. For example, people can have temporary disabilities, such as a broken arm or lost glasses. Or people can encounter situational limitations, such as bright sunlight when using their smartphone.
Accessibility is part of the whole process of creating a website, from designing a website to building it. Considering accessibility from the start of the project leads to improved overall usability and a better web for everyone.