I'm a bit confused by your question - is it that you don't understand what a percentile is, or is it that you specifically don't understand why you see 'NaN%' after you gave a (gmatclub) test. I'll try to explain both.
An example will help you understand this easily.
Let's say you got 85%ile in a math test.
This means that your score is above 85% people who have appeared for the test. This, in other words, means that you are among the top 15% scorers in the test.
Now for the second part, if you saw the 'NaN%' message after you gave a gmatclub test, then that's because your gmatclub session timed out/expired before you finished your test. That is the reason why you couldn't see the exact percentile.
However, you can see the results if you click the 'View your results' option available on the
GMAT Club Tests page.
Hope this helps!