What is web accessibility? This is a difficult question to answer, but my short answer is:
An accessible web page makes all pertinent content available to any person who wants it regardless of their abilities or browsing technology.
Web accessibility benefits more than people with a disability; keep in mind that a user may be situationally disabled, temporarily disabled or using less performant technology.
The W3C’s description says:
Web accessibility means that websites, tools, and technologies are designed and developed so that people with disabilities can use them.
Here is some terrific further reading: