A11y is short for "Accessibility", the practice of making sure your site or app is accessible to all users, regardless of their device or impairments. Things you should keep in mind when designing a web site or application is to make sure that people with visual impairments can use your product, by making sure text sizes are large enough, colors have enough contrast and that you don't depend on color alone to communicate meaning (for people with color blindness).
When building a site, things you should keep in mind are making sure the site is accessible for people using just a keyboard or other types of input mechanisms, that buttons and links are large enough to be clicked or tapped, and that images or videos have alternative text content that provides the same information.
There are many more things to keep in mind if you want to make an accessible site or app. A good start is the WCAG, the Web Content Accessibility Guidelines. You can find a good intro on WCAG at MDN.