What it means the ping number when testing the internet speed a 31ms number is good?

Yes. if you are pinging to destination on the internet then 31ms is good. "ms" is the time it takes for your ping (test packet) to get to the destination and back in milliseconds (1/1000 second). This is called the Round Trip Time (RTT).

If you were pinging a computer in the same building, then 31ms would be bad (slow). You would expect it to be about 1 to 10ms

If you were pinging a destination on the other side of the world, you would expect a RTT of about 300ms +/- 100ms.