The ping (or latency) of a player is how fast they can send a command to the game and have the game (or its server) display what happened.
So in an environment where the slightest misstep could have disastrous results for you and your teammates, low ping is a necessity.
What’s a good ping?
An acceptable ping is around the 40ms-60ms mark or lower. A speed of over 100ms shows a noticeable delay and over 170 some games will reject your connection entirely.
If you have, say a 10ms ping (0.01 seconds), your gameplay will seem faster and smoother than playing with 100ms, for example.
It really depends on what game you are playing. A game like Hearthstone may not require a low ping, as it is a slower turn-based game. Other titles like Counter-Strike and Street Fighter V require precise inputs and timing, which would require a much lower ping to play at the top level.
What is lag?
Lag is often a result of high ping and/or a low speed internet connection. It’s where the game plays out your commands with a delay. So for example, you press W to move forward in Counter-Strike, but your character doesn’t actually move until a second later, for example.
Sometimes lag can build up, so the game will seemingly freeze for a few moments, then suddenly catch up with all your inputs in one go, or disconnect you from the game entirely.
Gamers can experience lag for a multitude of reasons, the main one being high ping, where your commands are being sent to the game on a delay. Sometimes the problem is instead because of unstable connections, where your device will periodically disconnect and reconnect to the internet, this is called packet loss and occurs when the information you send to the server in packets is not received. This can be due to an over crowded server, low security or internet disconnections.