The phrase "head curtain" refers to a safety feature in cars. It is a protective curtain that comes down from the roof, typically above the windows, in order to shield the heads of passengers in case of a collision or accident. Full definition
Beginning with 2005 models, front and rear side head curtain airbags became available as options. (iihs.org)
In the 2009 model year, front and rear head curtain side airbags became standard except in cars sold to fleet buyers that could still be ordered without side airbags. (iihs.org)
If the vehicle is equipped with seat mounted side impact or roof rail mounted head curtain air bags, those will deploy in a side impact. (gmc.com)