Broadly speaking, when we say a site is accessible, we mean that the site's content is available, and its functionality can be operated, by literally anyone. As developers, it's easy to assume that all users can see and use a keyboard, mouse, or touch screen, and can interact with your page content the same way you do. This can lead to an experience that works well for some people, but creates issues that range from simple annoyances to show-stoppers for others.
https://developers.google.com/web/fundamentals/accessibility/focus/
https://developers.google.com/web/fundamentals/accessibility/semantics-builtin/
https://developers.google.com/web/fundamentals/accessibility/semantics-aria/
https://developers.google.com/web/fundamentals/accessibility/accessible-styles
https://developers.google.com/web/fundamentals/accessibility/how-to-review