Multiple machines sharing a network and talking at the same time cause collisions. When the collisions are detected, both machines stop sending, wait a random amount of time, and then try to transmit again.
CARRIER SENSE MULTIPLE ACCESS/COLLISION DETECTION (CSMA/CD)
Meaning of CARRIER SENSE MULTIPLE ACCESS/COLLISION DETECTION (CSMA/CD) in English
Josh's Network English Glossary. Английский словарь Josh's Network. 2012