Web accessibility means that those with disabilities (visual, auditory, physical, speech, cognitive, and neurological disabilities). can perceive, understand, navigate, and interact with the web, as well as contribute to the web. It also benefits those with changing abilities due to things like aging. Accessibility also helps in designing websites that are flexible to meet different user needs preferences and situations.

Web accessibility is important as it provides equal access and equal opportunity, as well 6as help people to more actively participate in society with the possibility of unprecedented access to information and interaction.

The responsibility of creating web accessibility lies on web developers aided by web software. Making it accessible can be either simple or complex depending on factors such as type of content, size and complexity of the site and the development tools and environment.

Many accessibility features are easily implemented if they are planned from the beginning of Web site development or redesign. Fixing inaccessible Web sites can require significant effort, especially sites that were not originally “coded” properly with standard XHTML mark-up, and sites with certain types of content such as multimedia.

When developing or redesigning a site, evaluating accessibility early and throughout the development process can identify accessibility problems early when it is easier to address them. Simple techniques such as changing settings in a browser can determine if a Web page meets some accessibility guidelines. A comprehensive evaluation to determine if a site meets all accessibility guidelines is much more complex.

Reference: https://www.w3.org/WAI/intro/accessibility.php