PowerBI allows users to add measures to tooltips. Many users want to add text columns as well. Today if you drag a text column to the tooltips, a count measure is created which is not very useful. Before PowerBI enhances the tooltip feature by allowing users to drag text columns to tooltips to show the text contents, you can make it happen today by creating DAX measures. Let’s say you want to add ‘Table'[TextColumn] to tooltips. Create a measure as below.
[TextColumn Tooltip] = TOPN(1, VALUES(‘Table'[TextColumn]), [TextColumn], ASC)
Now you can drag the newly created measure to tooltips. I have created a sample model with such a tooltip based on AdventureWorksDW database. As you can see in the picture below, when users hover over an individual product, the tooltips include the [Description] column for that product.