If you are referring to the "show port congestion" output it is referring to packet drops on the device. This command is used as a troubleshooting tool and accumulates over the up time of the device. If the counters increment on a port then there is typically some type of issue and would have to be investigate per incident using other tools on the device. If you run the command and the counters do not increment then it is impossible to find out why or when there were drops.
