As the free electrons move through a solid they collide with the atoms of the material and this affects their motion causing resistance.
Some materials have more free electrons than others and so these materials have a low resistance.
A high resistance means that the free electrons will make lots of collisions and lose lots of energy.
You can compare this with a person struggling to get through a large crowd; they will also lose lots of energy as they collide with the people in the crowd.
