They are testing a wind sensor to detect changes in wind speed and direction from large distances. This is part of the Adaptable Lighter-Than-Air (ALTA) balloon program. The balloons can change altitude and position to stay in one spot.
DARPA balloons could provide secure communications and navigation or act as a mother ship for drones.
OAWL shines pulses of laser light into the air. A small fraction of the beam is reflected back, and the reflected laser light is gathered by a telescope. The wavelength of the reflected light is changed slightly depending on how fast the air it bounced back from is moving, a change known as doppler shift. By analyzing this shift, OAWL can determine the speed and direction of the wind.