EMZETT.CSMA/CD
In short: CSMA/CD (carrier sense multiple access with collision detection) was the access method of classic Ethernet.
In more detail: A device listens whether the line is free. If it sends and notices a collision at the same time, it stops, waits a random time and sends again.
In Depth
Today
In modern switched networks with full duplex there are no collisions any more, so CSMA/CD is practically no longer needed. It applied to hubs and coax cable (hub, coax).