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.
Fire resists Grass and is super effective against Grass, Water resists Fire and is super effective against Fire, and Grass resists Water and is super effective against Water. In other words they make a type triangle.