Yikes! Came outside and found crop circles.
Not really! Just kidding, those are Painted Circles, done with an aerosol can of black paint and a 12" circle tool.
The rectangular outline is 4-1/2" x 5-1/2", which corresponds to the end size of three 2" x 6" pieces of dimension lumber - the size of our "posts".
The 2 holes correspond to the back side of a "post" and are used to measure and align jig.
To use it, first establish a "zero" point at one of the string corners and simply work from there by measuring the appropriate distance from post edge to post edge. Use a plumb bob along the string to ensure the back edge hole is at the proper distance from "zero" and drive a 6" spike into the ground. Set the jig over the head of the spike. Then plumb bob the front edge of the rectangle to ensure it's parallel with the string also.
Here's what it looks like with the circles done on one side. They really are in line, but the camera operator didn't get lined up, so the alignment isn't all that obvious.
Why go to all that trouble? Well, I've dug postholes "freehand", by eye before and usually there's one or two holes that are "off" and you've got to pull the pole out and dig again. This method should eliminate that kind of error.