Think of ice as a highly ordered crystalline solid in which the water molecules are able to bind strongly with each other. How strongly the molecules bind to each other determines how much energy, in the form of heat, will be required to break them apart.
Whether its ice or an altogether different type of solid, what happens when you disrupt these inter-molecular forces (i.e. hydrogen bonding, van der Walls forces, etc.) by introducing a second chemical substance? It requires less energy, less heat, to break apart the solid. Thus, it’s melting point is lowered. This is commonly known as "freezing point depression".