...
To obtain the geopotential height (h) in metres (of the land and sea surface or at particular heights in the atmosphere), simply divide the geopotential by the Earth's gravitational acceleration, 9.80665 m/s2. This geopotential height is relative to mean sea level - for more information see ERA5: data documentation. Note, in the Climate Data Store (CDS) for ERA5, geopotential at the surface is called orography.
...