The Line on the Meteog Graph

I was reading the help file on this but I couldn't understand what this line that I highlighted is on the Metog. graph.
Can you tell me in simple terms what I'm looking at?
Thanks.

This curve shows the variation in atmospheric pressure. In relation to the pressure values on the Pressure line

@idefix37 what's confusing me is the altitudes on the left. Should i be ignoring them? I'm not sure if this is a plot on a different axis or the altitudes mater.
It seems like it would make more sense to me to show what altitude a normal (normalized sea level) pressure is at.

@jimerb
These altitudes on the vertical axis allow to know the altitude of clouds shown in the Meteogram.
They are expressed in pressure (hPa=h), altitude (km) and flight levels (FL or ft). You know that pressure decreases with altitude.
Pressure at sea level is shown in the Pressure line (highlighted in red in my screenshot) 
@idefix37 thanks for the clarification!
