[Surface tension][1].

A molecule/atom of the liquid minimises its energy by being surrounded by other molecules/atoms of the liquid. At the surface this isn't possible so the surface has an excess energy, and this interfacial energy produces a force called surface tension that tries to reduce the area of the surface. There is a fuller discussion of this in my answer to http://physics.stackexchange.com/questions/274076/why-is-sticking-to-glass-a-lower-energy-state-for-water.

The tendancy to reduce the surface are is why liquid droplets are round, because spherical shape minimises the surface area for any given volume. It also means the surface will be smooth because any roughness increases the surface area and therefore increases the energy.

The surface will not be precisely smooth because we expect that thermal vibrations will produce deformations of the surface with energies of the order of $kT$ per surface molecule. At a macroscopic scale these deformations are too small to see, but at the atomic scale the surface will be fuzzy rather than sharply defined.


  [1]: https://en.wikipedia.org/wiki/Surface_tension