A sharp and experienced programmer who creates program code either many times faster than the average programmer or with much greater sophistication. Of course, the code must be bug free or almost bug-free. The 10 factor is just an expression of "greater than" and not a hard and fast metric.
There Is a Difference
Like any profession, there are highly skilled workers and people that just get by. Some programmers can manipulate code like an experienced typist moves words around. However, to be a true 10x programmer, the code must be documented well enough for others to readily change if required in the future. See
coder and
programmer.