[ Fffff this is like KH all over again HERE, DERPY PROTAG, THIS IS FIGHTING except lol it's pokemon. ]

A resistance is the opposite of a weakness; attacks from a type your pokemon resists won't be as effective. Fire-types are resistant to Grass, and Steel-types are resistant to... [ bad example fff ] ...well, lots of things.

Immunities are exactly what they sound like; there aren't many, though.