wonderful web page and it gets better. I am about to sail from Tahiti to Fiji and we will use windy to sort our weather and course routing before and during the trip in late May or early June.
Pressure in hPa (hectoPascal).
The "standard" pressure on the surface (sea level) is 1013 hPa.
As we go up, the pressure falls.
So, at 850 hPa we are at elevation of 1500m (5.000 ft)
at 500 hPa, 18.000 ft etc