The thundering herd problem occurs when a large number of processes waiting for an event are awoken when that event occurs, but only one process is able to proceed at a time. After the processes wake up, they all demand the resource and a decision must be made as to which process can continue. After the decision is made the remaining processes are put back to sleep, only to wake up again to request access to the resource.
This occurs repeatedly, until there are no more processes to be woken up. Because all the processes use system resources upon waking, it is more efficient if only one process is woken up at a time.
This may render the computer unusable, but it can also be used as a technique if there is no other way to decide which process should continue (for example when programming with semaphores).
Though the phrase is mostly used in computer science, it could be an abstraction of the observation seen when cattle are released from a shed or when wildebeest are crossing the Mara River. In both instances, the movement is suboptimal.