The answer is There has been a data collision.
In carrier-sense multiple access with collision detection (CSMA/CD) networks, the jam signal indicates that a collision has occurred.
Whereas, In CSMA/CA or carrier-sense multiple access with collision avoidance networks, the jam signal indicates that the sending station intends to transmit.
A jam signal informs all the devices that a collision occured,so for all the devices collision invokes a random backoff algorithm then each device on the ethernet segment stops transmitting for a short time until the timers expire.
