Ping is a unix command (though also available on MS Windows) that helps determine the connectivity between two machines. It sends a small packet of data to a remote host which is expected to return it. Then the ping program can report how long did it take for the data to arrive back (the round-trip time) if it arrived at all.
In order to determine if a packet has been lost or it is just taking a looong time to come back, the utility employs a timeout mechanism. If a packet does not return within the given timeout period, it is considered as lost.