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For more details about the vertical discretisation in the ECMWF Integrated Forecasting System (IFS), please see Part-iii Dynamics and numerical procedures, section 2.2 and the FULL-POS documentation at:
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In ERA5, and often in meteorology, heights (the height of the land and sea surface, or specific heights in the atmosphere) are not represented as geometric height, or altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid, which is represented by the mean sea level in ERA5). However, Note that ECMWF archive the geopotential (in m2/s2), not the geopotential height.
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, which has a fixed value of 9.80665 m/s2 in the IFS. 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.
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