The purpose of this Garmin Connect IQ widget is to indicate whether your Garmin has a good GPS fix. It only makes sense to use this widget when you are stationary.
The widget plots your most recent GPS positions in a rectangle that is 20m wide. It then draws the smallest circle that will fit around your points. The smaller the circle, the more certain your Garmin is of its position.
The widget then computes the greatest distance between your most recent positions and displays the value against the "Accuracy(m)" label.