This parameter (together with error_burst) is used to limit how many ICMP destination unreachable to send from the host in question. ICMP destination unreachable messages are sent when we can not reach the next hop, while trying to transmit a packet. It will also print some error messages to kernel logs if someone is ignoring our ICMP redirects.
The higher the error_cost factor is, the fewer destination unreachable and error messages will be let through. The default settings limit warning messages to five every second.