Duty to mitigate refers to the obligation of non-breaching party to do whatever it can (in its limit) to prevent any other additional losses to the other party after the breach of contract hast taken place.
In case the aggrieved party fails to carry out this duty, the avoidable losses compensation may not be given to the aggrieved party. An employee who was wrongfully removed from employment should mitigate damages by getting onto another job. They can't simply be idle and sue the employer for losses.