There are four guiding principles of accessibility that government regulations and industry standards are based around:
If people don't even know your content exists, they're not going to be able to use it. It means providing text that can be read aloud by software for people who can't see an image or adding captioning to a narrated video for people who can't hear.
Most websites and software products are designed to be navigated by a mouse, or these days, a touch screen. But what happens when people can only use a keyboard or have to use voice commands to control their phone? Fancy animated menus and whizbang special effects all too often create obstacles for people who can't operate them.
Making content perceivable and operable doesn't help if the users aren't able to understand it. Instructions should be clear, navigation should be simple, and writing should be easy to comprehend.
The kinds of technology people use to help assist them online works best when websites are built according to standards and best practices. While you don't have to keep every kind of technology in mind, making sure the basics are covered means