Ping is used to test whether a particular host is reachable across a network. It works by sending ICMP packet which in effect is an echo request to the target host and listening for the reply. Ping estimates the journey and response time usually in milliseconds, it will also display connectivity issues such as packet loss.

NON GEEK VERSION - FOR PEOPLE WHO NEED INSTRUCTIONS TO WORK TOASTERS !!
Ping when one system shouts down the line 'hello are you there' and another system responds with 'yes, I am here' the time sending this message and receiving it, is in it simplest form ping, if the connection is dropping and you only receive part of the message this is called packet loss