Polar Mode Hint Window
When adding points in polar mode, the point is forced to fall on a line that is a multiple of 90 degrees from the previous point. This is most commonly used to create horizontal or vertical lines. It can also works for lines of various angles. You can select the desired angles in the User Preferences Window.
Novice users may inadvertently turn on polar mode then not understand why points are being moved. To aid them, the Polar Mode Hint Window is displayed.
This window will disappear when you exit polar mode. This can be done by clicking (toggling) the Polar Mode button in the Measurement Toolbar or by pressing the "End Polar Mode" button in the hint window.
Experienced users may not want this hint window. To disable it, Uncheck the box "Display this hint whenever in polar mode". To re-enable it see the "Hints" tab of the User Preferences Window.