ARIA (Accessible Rich Internet Applications) is a set of HTML attributes that give assistive technology (AT) extra context about your UI. You add them to your markup to help users with disabilities better understand and navigate your app.
This cheatsheet isn't a comprehensive specification. It's a quick reminder of the things you'll reach for regularly: which roles go where, which attributes to add, how users navigate with a keyboard, and what happens when elements are toggled, selected, or activated.
When this page doesn't cover what you need, the WAI-ARIA Authoring Practices Guide has the complete official specification.