Sometimes a MarkLogic Server database will have several forests. If the forests are present from the time when the database was created, they will each have approximately the same number of documents. But, if the administrator adds more forests later on, the newer forests will tend to have fewer documents. In cases like this, we can rebalance the forests.
See the github repo for more details.
Note: If you are using software that pre-dates MarkLogic 7, you may find this project useful. As of version 7, MarkLogic comes with rebalancing built-in.