Traceroute
A tool that shows every router hop a packet takes on the way to its destination.
traceroute thern.io sends packets with increasing TTL (time-to-live). Each router that drops a packet due to TTL=0 sends back an ICMP error — you see every hop.
Useful for diagnosing "why is my connection slow?" — you see where the latency spike happens. On Windows the command is tracert.