Most browsers are built with keyboard-only functionality for those who either can’t or choose not to use a mouse. To assist those users, your site should allow users to move through the page via the keyboard in an order that makes sense and lets the user know exactly where they are on the page. Keyboard focus visibly informs the user of their current location.
Focus order is the navigation sequence of the elements on a page. Good focus order allows the user to move from one element to another in a way that makes sense and preserves meaning and operability.