The Solar Heat Gain Coefficient measures the amount of solar radiation a window permits to pass through. Essentially, it measures how much the sun is able to heat your home by shining through the windows. The lower the SHGC, the better a window protects your home from solar heat.
Based on your climate, you may want a window with a higher SHGC rating to allow solar heat to help warm your home in the winter. In Central Pennsylvania, we have hot summers and cold winters, so a high-performance window featuring Low E glass and a mid-range SHGC may be a better fit and provide better heating and cooling protection for your home.