A ping flood is a simple denial-of-service attack where the attacker overwhelms the victim with ICMP "echo request" (ping) packets.
[1] This is most effective by using the flood option of ping which sends ICMP packets as fast as possible without waiting for replies.
Most implementations of ping require the user to be privileged in order to specify the flood option.
If the target system is slow enough, it is possible to consume enough of its CPU cycles for a user to notice a significant slowdown.
A ping flood can also be used as a diagnostic for network packet loss and throughput issues.