Accessibility, sometimes shortened to A11y (because there are 11 characters between the “A” and the “y”), refers to efforts made to ensure that resources are usable by all people. In the context of digital and web development, especially, this means making sure that sites and apps work for people who have impaired hearing, vision, mobility, and other circumstances.
Accessibility is often referred to in the context of Section 508, the part of US Law which establishes standards. While federal, state, and local governments often lead the charge in delivering accessible resources, all developers and designers should be conscious of the diversity of needs and abilities among their audience/userbase.
The News Nerdery slack has a channel for discussing #accessibility. (See the NN website for info on how to request access to the Slack)