What are collisions ?
When two network devices attempt to transmit information at the same time, the signals overlap with each other, creating a collision. Collisions are a normal part of Ehternet networks, and not considered errors.
However, under heavy load condition, too many packet collisions will greatly reduce the whole network efficiency due to the collision and retransmission mechanism.
Using switches rather than hubs in the network reduces packet collisions: packets are checked for errors, held and forwarded by the switch to a particular node when lines are clear.