Impurities disrupt the repeating pattern of forces that holds the solid together. So a smaller amount of energy is required to melt the part of the solid surrounding the impurity. If less energy is required, then this explains the melting point depression (lowering) observed from impure solids. The more impure the solid is, the more the structure is disrupted and the greater the variation in intermolecular forces in different areas of the solid. There is an explanation video available below.