This is the number of collisions that were encountered while trying to transmit frames.
The NIC checks for a carrier sense when it knows that the network hasn't been used for a while, and then starts to transmit a frame of data. The problem occurs when two network cards check for the carrier sense and start to transmit data at the same time. This error is more common on busy networks.
When the NICs detect a collision, they stop transmitting and wait for a random period of time. The time periods are different for each NIC, so in theory, when the wait time has expired, the other NIC will have already transmitted or will be still waiting for its time to expire, thus avoiding further collisions.
You can reduce this type of problem by introducing a full-duplex network.