Tooltips appear over a visualization when you hover over it, providing additional information and/or information in a different format. The report creator can turn off tooltips. Power BI has a 2 types of tooltips
You can also try out some tooltips at the bottom of this page.
Default tooltips will show the data that is already in the visualization.
For example if you have a pie chart where the size of the slices represent a number and the colour of the slices represent the day of the week, then when you hover over a slice the tooltip will show the day of the week for that slice and the number for that day of the week.
Example Default Tooltip
The report creator can also add additional values to the default tooltip. For example, if we add "earliest date" to the previous tooltip example then when you hover over the Thursday slice in the pie chart, it will show you the earliest Thursday in the dataset.
Example Default Tooltip with Added Field
Lastly, you might sometimes see an additional message saying "right click to drill through" this only appears if a drill through page on one of the variables in the visualization was created. Drill-throughs will be covered in the next section "Drilling".
Example Default Tooltip with Drill-through message
Check out this Video example to see how the tooltip text changes when the mouse moves over different parts of the visualization
Custom tooltips are created by using a seperate report page, so they can be designed in any way by the report creator. Likely they are 1 or 2 visualizations. When you hover over a mark in the base visualization, the visualization in the tooltip will filter to show only data represented that mark. For Example if I have a line chart with a number over months and years as my base visualization and a bar chart with a number over days of the week as the tooltip, then when I hover over a point on the line chart that represents a month, say June 2022, then the bar chart in the tooltip will represent days of the week in June of 2022.
Example
Check out this Video example to see how the tooltip text changes when the mouse moves over different parts of the visualization