it is the amount of heat released when an alkene is hydrogenated.
more the heat of hydrogenation less the stability of the alkene.
the diagram inicates the energy levels of the two isomers 1 and 2 and the final product P.
the second isomer has a greater heat of hydrogenation than the first.
from the diag. energy of the second greater than energy of the first.
so the second isomer is less stable than the first.