Question

What is ICMP and what is it good for?

Answer 1

The Internet Control Message Protocol (ICMP) is a protocol for sending IP-layer error and status messages.

ICMP is communicated from host-to-host similar to Internet Protocol (IP)

Some common ICMP types are:
Echo Request
Echo Reply
Destination Unreachable
Source Quench
Redirect Message
Router Solicitation
Bad IP header or Parameter
Timestamp reply

Answer 2

ICMP is the internet control message protocol. It sends error messages and information showing that a request or requested service was not available, or the host or router was not reached