What does steering angle mean? I'm reading the UAVSAR data format and in File Naming Convention section is said that if we take a typical file name:  

Dthvly_34501_08038_006_080731_L090HH_XX_01.slc  


as the name that follows  

Dthvly_34501_08038_006_080731_L  (090)  HH_XX_01.slc  


The three digits that are in parentheses, is the steering angle.
And for the above file name, we have:  


  Dthvly is the site name, 345 degrees is the heading of UAVSAR in
  flight, with a counter of 01, the flight was the thirty-eighth flight
  by UAVSAR in 2008, this data take was the sixth data take during the
  flight, the data was acquired on July 31, 2008 (UTC), the frequency
  band was L-band, pointing at perpendicular to the flight heading (90
  degrees counterclockwise), this file contains the HH data, this is the
  first interation of processing, cross talk calibration has not been
  applied, and the data type is SLC.  


What is steering angle?  

Please explain very simply because I'm not very familiar with aviation concepts. I just want to have a sense and intuition about of the aircraft steering angle.
Steering angle, according to the text you bolded:


  pointing at perpendicular to the flight heading (90 degrees counterclockwise)


means the steering angle 090 is the angle, relative to the flight heading counterclockwise, that the data sample was taken.

If the flight heading was 345 degrees, 90 degrees counter clockwise (CCW) is 255 degrees.  This steering angle has no aviation context and is just a relative offset from the aircraft heading for data acquisition (which in this case sounds like a radar sample).