PostCSS at-root is a plugin that allows CSS rules to be emitted at the root of the document, rather than being nested beneath their parent selectors, using the `@at-root` directive. This is particularly useful for developers working with PostCSS who need to manage CSS rules that should not be scoped within a specific selector, such as viewport rules or global styles. It integrates seamlessly with other PostCSS plugins, making it a valuable tool for CSS preprocessing workflows.