The quantity that is actually computed at each grid point is the two-dimensional wave energy spectrum and the total surface stress for a given forcing by surface (10 metre) winds. The two-dimensional spectrum describes how the mean sea-surface elevation variance due to ocean waves is distributed as a function of frequency and propagation direction. The energy spectrum is discretised over a number of frequencies and directions. Read more about 2D wave spectra.
http://www.ecmwf.int/en/what-are-2d-wave-spectra