Energy from the sun travels to the earth by electromagnetic waves which range from infrared (longer and slower than visible light) to ultraviolet (shorter and faster than visible light). What happens when these waves strike a surface depends on the properties of that surface.
If the surface is reflective, (i.e. polished metal), most of the sunlight will be reflected off it. If the surface is translucent (i.e. a window), much of the sunlight will pass through, although some will also be reflected. If the surface is opaque (i.e., not translucent) much of the sunlight will be converted to heat energy and absorbed by the material.
One substance which enhances our ability to capture this solar heat is glass, which allows visible light to pass through it, but absorbs and reflects a certain amount of heat. When visible light goes through glass and then strikes an opaque object, heat is produced (all of which cannot then radiate back out through the glass). The action creates what is known as the "Greenhouse Effect," something familiar to everyone who enters a closed car which has been sitting in the sun.
The amount of heat available from the sunlight striking the surface of earth is tremendous; approximately 200 Btus per square foot per hour. But there are difficulties in storing this heat for use. In the first place, availability and demand are unbalanced: more solar heat is available in the summer when it is not needed than in the winter, when it is needed. None is available during the night and in stormy weather when demand may be high. Second, not all of the sunlight available can be collected and/or converted. Enough "useable" heat could still be produced to reduce the amount of energy purchased by the resident.
The conversion of sunlight to heat requires a collection device. The simplest solar collector is a south facing window behind which the structure and furnishings of the room serve as the opaque surfaces which convert light to heat.
All collection devices should include: (1) "collecting" surface in position for the sun to shine on it; (2) a surface to convert a maximum amount of sunlight that strikes it to heat; (3) "mechanism" for transferring the heat from the collection surface to a desired location for use or storage; and (4) a way to reduce heat losses from the system.