Ping is a measure of responsiveness and quality of connection to a remote computer. Officially wikipedia:Ping is a network diagnostic tool and it measures latency and packet loss. In Warhawk, Ping usually refers just to the latency, the number (in milliseconds) shown in your Server List that shows how delay-free your connection is to a game server in the list. It is presumably calculated by timing how long it takes for data to go from your PS3 to the server PS3. Though some games calculate a "ping" that isn't like "real" ping, it's a more sophisticated, game-specific "ping", and there's no way to know how Warhawk is calculating it.
The further away a server is, or the poorer your connection to it, the worse your ping/latency gets. If you have created your own online Player server, you will have 0 ping (since the server is running on your PS3).
A low ping/latency (0-100) is good...that means when you do something in-game, it is received and acknowledged very quickly by the game server, so your actions take effect immediately. Though even if your ping/latency is low, if your packet loss is high, you might still experience lag.
A high-ping/latency (over 300) is bad...you can end up with lag. Your actions take longer to take effect, and you don't get notified of other people's actions soon enough. With a high ping, you might fire a rocket, but by the time the game acknowledges that you've done so, your target will have moved. Or someone may fire a missile at you and you will never know it's coming, you will just die instantly.