While propulsion usually requires propellant of some description, a more “green” method relying only on light from the Sun itself.
Sails rely on the physical property of conservation of momentum. On Earth, we are used to seeing this momentum as a dynamic pressure from air particles blowing into a sheet when sailing, propelling a vessel forwards. Light is comprised of photons, which have no mass, but they do have momentum and can transfer it to a sail. As the energies of individual photons are very small, an extremely large sail size is needed for any appreciable acceleration.