The story of accessibility is very similar to physical spaces in the real world. A building, to be accessible to wheelchair users, for example, needs access ramps, more spacious elevators, and bathrooms with grab bars. A building only becomes 100% accessible when anyone can get access to any of the spaces available there.
The same thing happens with websites. To say that a website is accessible means that anyone on any device and with any type of disability can navigate the site with ease and without restrictions.
Many of the aspects that make accessible websites are implemented with ease when designed from the beginning of the project. It is much easier to incorporate accessibility into your website or application when you are starting from scratch, than trying to fit accessibility into a website that already exists. Think about the work...