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.
Defensively, Grass types have both good and bad points. While they have key resistances to Water, Ground, and Electric, they are tied with Rock in having the most weaknesses out of all types with five, although only Ice, Fire, and Flying are commonly used offensively.