ECMWF WAM is just the name of the wave model which is developed and run by the European Forecast Center ECMWF.
The other wave model available on Windy is the WW3 run by the NOAA.
These models give a prediction of the significant height of waves i.e. the mean height of the highest third of all waves, and their period i.e. time between 2 wave crests.
Additional information here: