Visio Tool Tips

The tool tip displays as a box adjacent to the icon that the cursor is resting over.
In this case it is the "Line weight" control icon.

Tool tips can be toggled to display or not display. Choose View > Toolbars > Toolbars and either check or uncheck the Show ScreenTips option.

Some shapes have control handles that allow you to easily modify the shape in different ways. Control handles have a green outline with a gray fill. Resting the mouse cursor over a control handle will display a tool tip that provides specific information about the function.

Resting the cursor over a control handle can also display a tool tip.
If you are using shapes with controls this is a great way to get a hint as to what a control handle might actually do. If you are creating shapes for other users, consider embedding control handle tool tip text. It will make using your shapes easier.