Thermal pads consist of a conductive layer, usually made of a material such as silicone, copper or aluminium. The pad is placed between the hot component and a heat sink, such as a fan or a cooling block.
The most important factor in thermal pads is thermal conductivity. The higher this is, the more effectively the heat can be dissipated. Basically, the thicker the pad, the better the thermal conductivity.
When installing the pad, you should ensure that the surfaces of the component to be cooled and those of the heat sink are free of dirt so that the transfer of heat can be guaranteed.