The energy required to remove an electron from the surface of a metal is less than that required to remove an electron from a free atom. Why?The electron is bound to the nucleus in an atom by electrostatic forces. How is an electron 'free' on a metal surface?
Work function measures how much energy is required to remove a conduction electron from a metal lattice. Ionization enthalpy measures how much energy is required to remove an electron from an atom.
Conduction electrons have already partly escaped from the influence of one single atom, and are now more weakly bound to a whole crystal of atoms.
Conduction electrons are 'free' in the sense that they are not bound to one particular atom. They can move around freely within the crystal of metal ions.