In altitude, rather than considering the pressure itself, meteorologists refer to the height (geopotential height) at which a constant pressure is reached. For example they refer to the height of the 500hPa pressure level.
In Windy you must select Geopotential heights in More layers..., at bottom of layers list. They are displayed as isolines and can be combined with other layers such as Wind, Temperature, Humidity...
Use the altitude slider to select the level.