A traceroute uses ICMP messages to obtain details about the route being used for each hop. Each device responds to the ICMP request and displays details such as the hostname, IP, and response time.
If a router doesn’t generate a Time-to-live exceeded response, the traceroute will not know anything about that hop. A hop that outputs * * * means that the router is not responding to the type of packet you were using.
While this is not very helpful, it is quite normal.