This integer attribute indicates if the element can take input focus (is focusable), if it should participate to sequential keyboard navigation, and if so, at what position. It can takes several values:

a negative value means that the element should be focusable, but should not be reachable via sequential keyboard navigation;

0 means that the element should be focusable and reachable via sequential keyboard navigation, but its relative order is defined by the platform convention;

a positive value which means should be focusable and reachable via sequential keyboard navigation; its relative order is defined by the value of the attribute: the sequential follow the increasing number of the tabindex. If several elements share the same tabindex, their relative order follows their relative position in the document).

An element with a 0 value, an invalid value, or no tabindex value should be placed after elements with a positive tabindex in the sequential keyboard navigation order.

You can consult this article to see a comprehensive explanation of focus management.
