CSMA/CD, or Carrier Sense Multiple Access with Collision Detection, is an access method used in Ethernet.
When a device is about to send data over the network, it will sense if another device is already transmitting. If there is indeed another device transmitting, the device checking will back off from communicating for a random amount of time before retrying.
If two devices check the network at the same time and don’t see any other devices transmitting data, they will both transmit at the same time. When this occurs there is a collision. When a collision is detected, both devices use a random time to back off before retrying.
The back off time is random since if both devices talk on the network at the same time and back off at the same time they will have another collision.