Passive solar is a concept that's widely used in permaculture building design to conserve energy and take advantage of natural pre-existing environmental conditions. Passive solar is one of many permaculture technologies that can be used to make buildings more efficient, and it allows the occupants to work with, rather than against, natural conditions.
There's not much to it, other than understanding the position of the sun and then working that understanding into the design of your buildings and structures. The name "passive solar" refers to the fact that there are no machines and no moving parts - the sun's energy is harvested passively.
Passive solar design takes advantage of the fact that the angle of the sun's rays changes at different times of day and different times of year.
Passive solar works by designing buildings so that they selectively allow the sun's rays into a building at certain times, and not at others.
The effects of passive solar can be amplified by incorporating other permaculture technologies like thermal mass and passive ventilation to capture, store, and circulate the natural warmth that enters a structure.
The angle of the sun's rays is different at different times of year. Passive solar design typically places windows, glass blocks, or other clear materials in places where the sun will penetrate the walls during the cold months, naturally warming the interior of the structure.
Those same walls and windows are designed to block or reflect the sun's rays during warm months, so that interior of the structure remains cool during the summer.
"I'd put my money on the sun and solar energy... I hope we don't have to wait 'til oil and coal run out before we can tackle that." -Edison
The angle of the sun throughout the day in different seasons depends on the latitude of the building's location. Generally speaking, the sun is higher overhead during warmer months, and lower along the horizon during the winter months. Obvious, the angle of the sun changes less the closer one is to the equator, and this technique is more effective in temperate climates.
There are several tools available to help you find the angles of the sun in your location. One good tool is the sun position calculator at suncalc.net.
In permaculture, we typically talk about passive solar energy as a factor in building design. This concept is worked into most permaculture greenhouses, and it is increasingly common in building design for homes, offices, and any other buildings.
Entire landscapes can be used to take advantage of passive solar energy. Some times bodies of water are used to reflect the sun's energy. And sometimes tall walls of trees and shrubs are used to create a "trap" which concentrates and holds the warmth of the sun within an area on the landscape.
There is a combined DVD and eBook product available called The Greenhouse of the Future. The book details several permaculture technologies, including passive solar, that are combined to create a greenhouse which produces food all year long in extreme northern climates. You can see a trailer video at that link above.
Another great resource is the book The Solar House, by Dan Chiras of the Evergreen Institute. Mr. Chiras covers passive technologies that can be used for heating and cooling a house with little or no help from fossil fuels.