Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases. When a substance (solid, liquid or gas) is heated , it expands and when it is cooled, it contract.
for example : The rails of railway track are made of steel. While lying the railway track, a small gap is left between the two successive length of rails. The reason is that the rails expand in summer. The gap is provided to allow for this expansion. If no gap is left, the expansion in summer will cause the rails to bend sideway. This may result in a train accident.