The Data Explorer is available for all cluster tier sizes directly from the Atlas UI (behind the COLLECTIONS tab in your cluster). It is a powerful feature that allows you to query, explore, and take action on your data residing inside MongoDB Atlas (with full CRUD functionality) right from your web browser. It also allows you to create indexes and examine your current index usage.

As well as allowing easy access to your data directly from the UI, actions performed via the Data Explorer are logged in the Atlas alerting window (use the Activity Feed on the left of the Atlas UI to see a log of all your Data Explorer actions).

See also the MongoDB blog post New to MongoDB Atlas - Data Explorer Now Available for All Cluster Sizes.

So when should we use MongoDB Compass?

While the Data Explorer allows you to query and explore your data directly from the Atlas UI, if you wish to perform more advanced queries on your data (including sort operations), you could use the MongoDB tool MongoDB Compass.

MongoDB Compass is a great GUI for viewing and manipulating your data. In addition to exploring your data in a visual environment, you can also use Compass to optimize query performance, manage indexes, and implement document validation. 

The Aggregation Pipeline Builder in MongoDB Compass provides the ability to create aggregation pipelines to process data.

See also Available Compass Editions and Capabilities of Compass Editions.