When the barrier is removed, the fast (``hot'') atoms collide with the slow (``cold'') ones. In such collisions the faster atoms lose some speed and the slower ones gain speed; thus, the fast ones transfer some of their kinetic energy to the slow ones. This transfer of kinetic energy from the hot to the cold side is called a flow of heat through conduction.
Different materials transfer heat by conduction at different rates - this is measured by the material's thermal conductivity. Suppose we place a material in between two reservoirs at different temperatures, as in the following figure.
Let us now measure the flow of heat through the material over time. Knowing the material's cross-sectional area and length, the thermal conductivity of the material is then defined as
Thus, for a given temperature difference between the reservoirs, materials with a large thermal conductivity will transfer large amounts of heat over time - such materials, like copper, are good thermal conductors. Conversely, materials with low thermal conductivities will transfer small amounts of heat over time - these materials, like concrete, are poor thermal conductors. This is why if you throw a piece of copper and a piece of concrete into a campfire, the copper will heat up much more quickly than the concrete. It is also why fiberglass insulation, and also feathers and fur, have air pockets - dead air is a poor thermal conductor, and so the air pockets aid in cutting back on the heat loss through the material.
Home insulation is thus a poor thermal conductor, which keeps as much heat in as possible. Instead of being rated in terms of thermal conductivity, insulation is therefore usually rated in terms of its thermal resistance, which is defined as
Materials which have a high thermal conductivity have, by definition, a low thermal resistance - they are poor heat insulators. On the other hand, materials with a low thermal conductivity have a high thermal resistance - they are good heat insulators. Good insulating materials therefore should have a high thermal resistance. In fact, the ``R'' value quoted for insulation is the thermal resistance (in British units).