ICMP
In plain terms
A protocol for network diagnostics and error messages.
Going deeper
ICMP carries control and error messages (like 'host unreachable') and powers tools like ping and traceroute. It's not for transferring user data.
Under the hood
Used by ping (echo) and traceroute; often filtered by firewalls, which can hide reachable hosts.
Examples
Ping, traceroute, 'destination unreachable', network diagnostics.
Think of it like...
Like the network's status-and-error notices rather than its cargo.