Serializer Implementation..

Serializer is implemented in terms of another primitive called "mutex", that has two operations to "aquire" and "release" it. At one time only one process can aquire the mutex, other processes trying to aquire it at that time will block untill the mutex is released by the process that has the mutex aquired.
BTW, if multiple processes were waiting for the mutex to be released.. which of those processes get to aquire the mutex depends upon a component called scheduler.