When used in P2 Server, dataset refers to a type of datasource that returns tabular data, and is specified during the configuration of that datasource.
In P2 Explorer, a dataset refers to an SQL query that has been created for a dataset datasource. This query returns data in a tabular format, which can be added to a page so that components can use the data for visualisation purposes.
Adding a dataset to a page in P2 Explorer
Datasets that have been added to a datasource in P2 Server, and are available for adding to P2 Explorer