Does it describe the contents of the questions to which it is applied? and is it unambiguous?
No. That a question is about a program for a competitive coding is irrelevant to the question itself. It is unambiguous to the point that competitive coding does not have more than one meaning, but I think that it's a meta tag.
Is the concept described even on-topic for the site?
Stack Overflow is about programming (or coding), so yes.
Does the tag add any meaningful information to the post?
None whatsoever. The question would be answered the same way irrespective of whether it had this tag or not.
Does it mean the same thing in all common contexts?
It depends. Competitive coding often has different criteria in different places.
competitive-coding has 83 questions, 22 of them closed.
Burn!