Rebuilding Journey History means applying the defined and saved Journey Stages to your data retroactively to try to algorithmically re-build the historical trend data for the Journey Stages.

For example, once you defined your MQA criteria, the Journey Builder can tell you how many Accounts are currently MQA. 

When you rebuild your Journey History, Engagio will evaluate your historical engagement data to calculate which accounts would have been MQA, allowing you a much richer level of analysis of your account Journey.

You can find the "Rebuild Journey History" feature the top right corner of the Journey Builder.

Once clicked, it'll bring this prompt, indicating that you're about to kick off a data-intensive process that may take hours to complete (depending on your database size and the complexity of your Journey Stages).