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.
Grass-types are very strong against Water (as per the Starter triad), and can also pose trouble for Rock and Ground types. However, they are likewise very weak against Fire, and take heavy damage from Flying and Ice types. Bug and Poison also are super-effective against Grass. Grass types often have high speed and/or evasion, which can sometimes circumvent elemental weaknesses.