Keyboard navigation is the path a user without a pointing device takes through a page: Tab moves forward through focusable elements, Shift+Tab moves backward, Enter activates links + buttons, Space activates buttons + checkboxes, arrow keys move within composite widgets (radio groups, listboxes, sliders).
WCAG 2.1.1 (Keyboard, Level A) requires every functional UI element to be operable via keyboard. Common failures: custom dropdowns built on <div> that can't be opened by keyboard, modals that lose focus when opened, slideshows that auto-advance and can't be paused.
The fastest accessibility test is "unplug your mouse and complete a task". Most keyboard issues become obvious in the first 30 seconds; full WCAG compliance takes longer but starts here.