... what is the meaning of the lines labelled in "xxxh" ?
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