The yellow line in the Polar Meter shows the approximate amplitude and direction that sound is arriving from. The blue line shows the currently selected microphone polar pattern. The Polar Meter can be particularly useful for determining the direction that bleed from other instrument is arriving from, and setting the polar pattern to best reduce the bleed.
Because the microphone has two capsules this allows the Polar Meter to determine approximately how far off-axis the sound. But there is not enough information to distinguish between say +45 degrees and -45 degrees, so the Polar Meter displays them as being equivalent and mirrors the image across the vertical axis of the meter.
In a highly reverberant environment the Polar Meter will be more circular showing that sound is coming equally from all directions.
Watch this video of the plug-in's basic controls and features to learn more.