half duplex ethernet gets collisions as a part of how it works; csma/CD. When there are a lot of collisions it affects performance. This message is telling you that the router is seeing an excessive number of collisions on that interface.
It can be caused by valid increased load. It usually happens at some bottleneck in the net. It can also happen when a host screws up and causes a data storm (unicast) or broadcast storm. You need to profile the data on the link using a sniffer or some such tool.
