Tab order is closely related to focus order, and in most well-built pages the two terms describe the same thing: focus should move through the page in the same order a sighted user would read it, generally left to right and top to bottom in Western languages. Tab order is determined by the position of elements in the underlying HTML source, not by their visual position on screen, which is exactly why CSS-based visual repositioning can silently break it.
Testing tab order is one of the quickest manual accessibility checks available and requires no special tools, just a keyboard and attention to where focus lands at each step.