The electrons are negatively charged and the nucleus of the atom is positively charged. So these electrons in the atoms are bound to the nucleus, but there are few electrons which are not bound to the nucleus. When enough energy is given to the electrons, these electrons come out of the orbit and are not in the influence of the nucleus anymore. These electrons which are not bound to the nucleus are called free electrons. Why are free electrons named so?. These free electrons are named so because they move around freely without being attached to the nucleus. These electrons do not experience any electromagnetic force.
These free electrons move through the substance and are responsible for carrying electric current and heat. There are more free electrons present in conductors.