The men are being trained in how to calibrate a GM (Geiger Mueller, geiger counter). In a calibration, you relate the meter readout to the actual exposure rate in mR[milliroentgens]/hr. The actual exposure rates are indicated on the floor and have been calculated based on the distance from the source and the known activity of the source. The source is the small metal capsule hanging from a string in the center of the concentric circles on the floor. It is towards the lower right corner of the photo. The source is the same height above the floor as the center of the GM tubes. The latter are the vertical cylinders clipped to the front of the bottom portion of the meter case. Notice that these tubes are aligned up on the concentric circles. See more at Lee D. Cady V.A. Photograph Collection and its finding aid.