The time in ms (1000ms=1s) for the data to make a round trip to the server. If you are gaming, that's ~ delay that you get from when you press a button to when the action happens. Humans can react in about 100ms (depending on a lot of factors, so it's a huge range really, but - in gaming terms, that's a good ballpark.)  So, if your ping is over 100, your opponent get your ping - their ping extra time to react. Depending on the game, that's either irrelevant or the difference between winning and loosing...  ¯\_(ツ)_/¯