What is accessibility?
As a design principle, good accessibility means people of all abilities can interact with whatever you're designing. It's usually focused on people with disabilities or other limiting conditions, but accessible design benefits everyone. Accessibility is related to usability, which refers to the ease-of-use and intuitiveness of your design. Using these two principles always results in better, clearer design.
Why is accessibility important?
Video games are a powerful medium for storytelling and immersion. But for players with disabilities or other limiting conditions, poorly designed video games can be a barrier to entry. Nearly everyone will experience some limiting condition in their lifetimes, from having a broken arm or broken headphones to worsening eyesight and mobility issues as they get older.