Accessibility for the web means making sure that all users, including those who have different abilities, can access and use information and applications.
The W3C (World Wide Web Consortium) outlines the "diversity of abilities" this way:
Auditory disabilities range from mild or moderate hearing loss in one or both ears (“hard of hearing”) to substantial and uncorrectable hearing loss in both ears (“deafness”).
Cognitive, learning, and neurological disabilities involve neurodiversity and neurological disorders, as well as behavioral and mental health disorders that are not necessarily neurological. They may affect how well people hear, move, see, speak and understand information.
Physical disabilities (sometimes called “motor disabilities”) include weakness and limitations of muscular control (such as involuntary movements including tremors, lack of coordination or paralysis), limitations of sensation, joint disorders (such as arthritis), pain that impedes movement and missing limbs.
Speech disabilities include difficulty producing speech that is recognizable by others or by voice recognition software.
Visual disabilities range from mild or moderate vision loss in one or both eyes (“low vision”) to substantial and uncorrectable vision loss in both eyes (“blindness”). Some people have reduced or lack of sensitivity to certain colors (“color blindness”), or increased sensitivity to bright colors. These variations in perception of colors and brightness can be independent of visual acuity.
So, barriers for web users can include, among many factors:
Complex navigation mechanisms and page layouts that are difficult to understand and use
Inconsistent, unpredictable and overly complicated navigation mechanisms and page functions
Missing visual and non-visual orientation cues, page structure and other navigational aids
Websites, web browsers and authoring tools that do not provide full keyboard support
Controls, including links with images of text, and other images and structural elements that do not have equivalent text alternatives
Video content that does not have text or audio alternatives or an audio-description track
Web browsers and media players that do not provide mechanisms to suppress animations and audio
Text, images and page layouts that cannot be resized, or that lose information when resized
Text and images with insufficient contrast between foreground and background colors