Rollup is a multi-dimensional aggregation used to perform hierarchical or nested calculations. For example, if we want to show the number of records for each State+Year group, as well as for each State (aggregating over all years to give a grand total for each State irrespective of the Year), we can use rollup as follows:

scala> statesPopulationDF.rollup("State", "Year").count.show(5)
+------------+----+-----+
| State|Year|count|
+------------+----+-----+
|South Dakota|2010| 1|
| New York|2012| 1|
| California|2014| 1|
| Wyoming|2014| 1|
| Hawaii|null| 7|
+------------+----+-----+

The rollup calculates the count for state and year, such as California+2014, as well as California state (adding up all years).