When the NIC is about to transfer a frame, it checks first to make sure that it has carrier sense (much like before you dial the phone, you check to make sure you have a dial tone). While the NIC is transmitting the frame, it listens for possible collisions or any errors. These errors occur when a NIC is transmitting a frame on the network, and it notices that it doesn't see its own carrier wave (much like when you are dialing a number on the phone and you can hear the dial tones being pressed).
These errors are caused by plugging and unplugging cables on the network and by poor optical power supplied to the Fiber Optic Transceiver (FOT).