Accessibility relates to designing things so that a person with a disability can interact with them. A product, service, or system is said to be accessible if a person with special needs can use it.
Websites are similar to buildings in this sense – either they are built to allow a person with a disability to access and navigate, or they are not.
Web accessibility is just that: people of all abilities should be able to access and interact with your website. Some people use assistive devices like screen readers. Websites must be compatible with screen reader technology if they are to be accessible.