Is it ensure, insure, or assure?
A host which has experienced a collision on a network waits for a amount of time before attempting to retransmit. A random backoff minimises the probability that the same nodes will collide again, even if they are using the same backoff algorithm. Increasing the backoff period after each collision also helps to prevent repeated collisions, especially when the network is heavily loaded.
An example algorithm is binary exponential backoff.