Solar heat gain?
The Solar Heat Gain Coefficient (SHGC) measures the sun’s heat that is transmitted through a window.
Windows with a low SHGC block the heat from intense summer sun, and cut air conditioning costs. This is generally best if you live in a hot climate or for any window that gets too much undesirable sun.
However, if you live in a cold location, a high SHGC could be preferable because it enables your home to capture free solar heat.