Every time a node advances its lastAppliedOpTime, the new optime will be added to a list of potential stable timestamps. The actual stable timestamp will be the greatest timestamp in this list that is less than or equal to the replication commit point. Timestamps can be removed from the list of ‘potential’ stable timestamps after it is less than the current stable timestamp. i.e. when it is less than the replication commit point and there is at least one other timestamp in the list greater than it that is also less than or equal to the replication commit point.
Setting the 'stable' timestamp will tell WiredTiger what is a valid timestamp to take a checkpoint at.