Technically speaking, accessibility is defined by the requirements laid out by Section 508 of the Rehabilitation Act of 1973, and it's often conflated with the Americans with Disabilities Act(ADA). In Canada, the  Accessibility for Ontarians with Disabilities Act (AODA)  is used. 

Non-technically, it's giving someone with a disability, like limited or no sight or hearing, the ability to experience all elements of your website. For example, someone without sight is not able to see images or form fields on your website, so they need alternative text to inform as to the content of each image. Someone without hearing cannot listen to a narrated video, so they need captions.