In a nutshell
CSMA/CD, the rule behind classic Ethernet, makes a station listen to the channel and start only if it's idle (carrier sensing). It keeps listening while transmitting, so if two stations started together before each other's signal arrived, both detect the collision at once, send a short JAM burst to make sure everyone discards the frame, and abort instead of wasting the whole transmission. Each then waits a random delay whose range doubles after every collision, so retries rarely clash again.