ARIA (short for Accessible Rich Internet Applications) is basically HTML's way of saying, "Hey assistive tech, I've got some extra context for you!" It's a bunch of attributes you sprinkle into your markup to help users with disabilities better understand and navigate your app.
This cheatsheet isn't trying to be the entire WAI-ARIA Bible—it's more like a sticky note stuck to your monitor that reminds you what aria-expanded actually does. It focuses on the common stuff you'll hit regularly: what roles go where, what attributes to slap on, how users are expected to move around with a keyboard, and what's supposed to happen when stuff gets toggled, selected, or otherwise activated.
When in doubt (or when this page isn't cutting it), the WAI-ARIA Authoring Practices Guide has all the official words and wisdom. But if you're mid-sprint and your brain just went poof, this page is here to save the day.