Ping is request message for ICMP (stands for Internet Control Message Protocol). This protocol is used by different network devices to check if a service or a device is available. Usually it is used for network diagnostics.
Ping monitoring allows you to keep an eye on your network infrastructure. Ping failure means broken connection to a network entity. It is handy to use Ping together with HTTP monitoring. In this case, you get additional information - for example, you know if the error is site-related, or the whole server has gone down. However, sometimes ICMP could be blocked for incoming requests, in order to prevent ddos attacks.