This story (maybe it should have been an epic with hindsight) is to re-implement query execution in DataFusion using a physical plan that supports partitions and parallel execution.
This will replace the current query execution which happens directly from the logical plan.
The new physical plan is based on traits and is therefore extensible by other projects that use Arrow. For example, another project could add physical plans for distributed compute.
See design doc at https://docs.google.com/document/d/1ATZGIs8ry_kJeoTgmJjLrg6Ssb5VE7lNzWuz_4p6EWk/edit?usp=sharing for more info