Use your knowledge of heat transfer methods to explain how under floor insulation reduces the loss of heat through the floor. I understand that if the floor of a house is insulated, the floor prevents heat loss or gain. This is thermal insulation. I also understand that this is because the distance between the particles in an insulator is greater than that of a conductor, but I need to put it into a scientific manner. Any information will be appreciated.

There are three methods of heat transfer that come to mind.
The first is conduction. Conduction is when the excited particles of one atom or molecule touch the excited particles in the atoms or molecules of another substance. This is how heat goes from one solid to another, or from single particles to other single particles, in typical usage.
The second is convection. Through convection, energy is transferred in the same way as conduction, but instead of a solid absorbing the energy, a fluid, such as a liquid or gas, absorbs the energy. In addition, the absorption of energy causes the fluid to move about, generating convection currents. These convection currents are common in ovens, the ocean, and even the earth's mantle.
Finally, radiation is a way that heat is transferred. This is when the atoms and molecules of the substance eject energized parts of themselves to increase entropy. Common radiation includes alpha, beta, and gamma decay, which you can learn more about here. This radiation is physical, and can be stopped by hitting other particles.
To conclude, the floor insulation prevents the escape of heat through all of these methods. The floor is a good insulator, as you mentioned, and is resistant to conductive heat loss. The floor separates the heat in the home from the air outside, which slows down the convection currents beneath the flooring. The floor blocks the radiation emitted in the building, sending it back before it can escape.
As far as your question on what in particular makes the flooring a good insulator, there are a few reasons. The first is that the insulation has molecules that are separated from each other with distance. The second is that the insulation has layers of material. Heat transfers from one object to another and across thin films slower than across single materials. Third, the structure of the material may be lattice oriented in a way that increases resistance to heat transfer. This means amorphous materials, like amorphous silicon, will be more resistant than an aligned block of silicon. Even little things, like the Thermal Hall Effect, can make a difference in the insulation a material provides.
https://www.condmatjclub.org/jccm-content/uploads/2015/03/JCCM_MARCH_2015_02.pdf