The typical ethernet-topology is a bus. All
stations have access to the medium at the same time. To avoid
collisions, ethernet uses
which means "carrier sense multiple access with collission detection".
Before sending, the station checks whether the medium is free. If it is,
it starts sending. It can happen, that two stations send at the same time
and a collision occurs. Then the stations stop sending and try again later.
The backoff algorithmus manages how long they wait.
In the simulation you see an ethernet with two stations sending. Adjust
the traffic-intensity and see how the backoff algorithmus reacts!