Look at the center of that circle, and notice the symbol which is a VOR, a VHF radio navigation station (actually a VORTAC), which sends out a signal that lets the pilot know which radial of that station he is on. If you draw a line from the VOR to the airplane location, where it crosses that circle determines what radial the airplane is on. Radials are in degrees from 0 to 360. If it's a VORTAC (as this one is) you also get range to the station.
The blue box has the name of the station (Sioux Falls) and frequency.
Look it up on Wikipedia for more details