I had the same problem just now, and the documentation page Create tooltips based on report pages in Power BI Desktop describes how to do this. Probably this functionality was not around in March 2018 (when the accepted answer was posted).

Steps

In summary the steps are as follows (assuming you already have a visualization):

1. Create a new page, name it Tooltip 1
2. On the Tooltip 1 page, go to Format → Page Information and switch on the Tooltip toggle
3. Go to Canvas settings and set the Type to Tooltip.
4. Create a text box via Home → Text box, fill it with the text you want to show when one hovers over your visualization.
5. Select your visualization
6. Click on Format → Tooltip
7. Set Type to Report page
8. Set Page to Tooltip 1

Now when you hover over the visualization, the text box from step 4 will be shown.

Additions

• You can mark the page Tooltip 1 as hidden, such that it does not show up as a tab on your report.
• You can also customize the tooltip size by setting the Page size of the tooltip page to Custom with suitable width and height values.
• As described in the link, instead of text you can also create a whole visualization.

You can use a transparent shape as an overlay on the text you want the hover over test to appear on

Turn Action On

Type Page Navigation

Destination None

Tooltip - Add the test you want to display on the hover

I know this post is very old but the best approach I have found currently (in 2025) is to use the button functionality to insert an (i) or (?) circle. I write the help text into a measure and then turn on the tooltip feature for the button under Action > Tooltip > fx = measure. Then when the user hovers over the (i) circle, they see the help text.