Microsoft Power BI Complete Reference

Field descriptions

Descriptions can be added to the measures and columns of a data model to aid users during report development. Once descriptions have been applied and the dataset has been published to the Power BI service, users connected to the dataset via reports can view the descriptions as they hover over the fields in the fields list. This feature is particularly useful in communicating the business logic contained in measures, such as whether discounts are included or excluded in the Internet Net Sales measure. 

Although field descriptions are recommended, particularly for measures that contain custom or complex logic, they are not a substitute for the formal documentation of a dataset. In most scenarios, the field description will ...