What does an asterisk/star in traceroute mean?

Traceroute sends packets to the destination with the field "time to live" (TTL) equal to the number of hops.

Every router decreases the value of TTL of an incoming packet and if it sees an incoming packet with TTL = 0 then drops it, otherwise it decreases the value and sends it further. At the same time it sends diagnosing information to the source about router's identity.

If router does not respond within a timeout then traceroute prints an asterisk. Lines 14 and 15 show that routers which drop packets with original TTLs 14 and 15 did not respond within timeout.

If a packet is not acknowledged within the expected timeout, an asterisk is displayed.

From http://en.wikipedia.org/wiki/Traceroute

However zyzzyva.site5.com did eventually respond which is why you have line 16.

The most common reason I see this is because the server is dropping UDP packets. If you use the -I flag, then it'll finish very fast with no asterisks.

Another reason is that typically 3 packets are sent to each machine, and some machines will only respond to the first packet no matter what.

If the problem is just a timeout issue, you can set the -w parameter to the number of seconds you want to wait; e.g., traceroute -w 10 google.com will wait 10 seconds instead of the default of 5 seconds.

Note: Some traceroute clients use milliseconds instead of seconds for the -w parameter.