Passive-solar design is essentially design-with-nature.  Because the sun rides lower in the sky during the winter if you are facing South (i.e. in the Northern hemisphere), winter solar gain (sun-heat) comes directly into the home through South-facing windows.  As long as that heat is exposed to interior "thermal mass" such as ceramic tile over concrete or other dense materials, the daytime internal heat can build up and be released gradually as the home gets colder at nighttime.

Air-conditioning systems are sized based on the "peak cooling load." A building will have its highest peak load during the hottest of the summer afternoon hours as solar gain builds up inside.  A well insulated home can repel much of that heat; passive-solar strategies can additionally manage that heat naturally by shifting the peak load to the evening hours while the outdoor temperature is falling. All that is needed is a sheltering overhang over each South-facing window to prohibit direct summertime solar gain.  Insulation (conservation) and passive-solar strategies can be balanced to find the optimal mix of energy cost, comfort, and daylight/views for a more "natural" architectural experience.