induration. 1. the process of hardening or being hardened.
1 They're weak against the Ice and Water types in reference of erosion, which cause this elements to the ground. 2  They're immune against the Electric type because in real life, the ground is a natural sink of electrons.  The Bug type resists them because many bugs like worms live inside the ground and many of them eat it.
The various elements also determines success or failure in battle. Dual element monsters typically have the combined weaknesses and resistances of both elements. But if (such as fire/nature) a monster would be weak to an element(in this case fire) AND resistant, the resistance overrides the weakness. Thus a fire/nature monster has only one weakness. This chart shows the results of using an attack in battle.