We all know accessibility is important, but what is it?
It's the practice of making web sites and applications usable by people with disabilities. It can be easily overlooked when developing, but that's the best and easiest time to implement accessibility features.
Accessibility helps people with visual, hearing, mobility, and cognitive impairments. Often by using Assistive Technologies (AT), like screen readers. And often by just making the web site more usable, by keyboard and having good contrast.
Making web sites more accessible not only helps people with impairments, it helps all users, helps search engines understand your content better, and helps your site to work on less optimal hardware and conditions. Also, there are many laws regarding accessibility.
Developing more accessible sites is a goal of mine for next year. MDN's accessibility page is a great place to learn more.