"Thermal mass" is a term used to describe the ability of building materials to store heat, or their thermal storage capacity. The basic characteristic of thermal mass is their ability to absorb heat, store it, and release it at a later time. A very common example of thermal mass in a home is a concrete slab.

During the day, the sun's light will warm a house and this heat will 'soak' into the thermal mass of the house.  This is particularly effective when direct sunlight falls on the surface of a concrete floor or wall, for example.  At night, as the house becomes cooler, the heat stored in the floor will naturally be released to warm the house.

During the winter, the low angle sun can reach deep into the house to warm the floor and walls.  In the summer, overhangs and louvered panels can be used to block the unwanted heat from entering the house.
 
Again, this ancient concept for passive heating and cooling has seen recent technological innovations.  New phase change materials store energy not by getting hot, but by changing from solid to liquid (melting).  When the temperature drops, the materials solidify again and give up their heat to the house.

Floors with higher thermal mass are particularly well suited to radiant floor heating and to solar hot water systems, since the floor can also store heat absorbed from the heating system and release it slowly.  The trick here is to know how MUCH thermal mass is needed.  Too much solar mass (a thick concrete slab for example) will cause the radiant floor system to respond only very slowly to heating and cooling demands.  This can cause cold and hot periods during the day. 

The LEAFHouse uses a dense fiberboard floor that is relatively lightweight.  Consequently, it warms up and cools off relatively quickly, improving our ability to control temperature, but it is still effective at absorbing and distributing passive solar heat.