Ping is the time it takes for your game to communicate with the server.
It's measured in milliseconds, or one thousandth of a second. So a ping of 35 would mean it takes 35 milliseconds to communicate with the server.
Generally pings lower than 150 are good. Higher than that will result in consistent lag.
Many things can affect ping. Location/distance between you and the server host, your connection speed/host's connection ping, lots of things.