A published dataset is the dataset which will be used in queries.

Once you publish a dataset, it can be queried by you and by anyone to whom you have given a permission.

If you already have a published dataset and you publish a new dataset, the existing dataset will become a background dataset. The published dataset will now be used in all subsequent queries. The background dataset may still be queried if a query was initiated before the newer dataset was published. 

See also draft dataset.