Take Dragonite, for example. It's already weak 4x to Ice, and if Forest's Curse is used on it, it then would have the Dragon / Flying / Grass type, which makes it 8x weak to Ice. Or practically, any Pokémon which is already 4x weak to the types to which Grass is weak (Fire, Poison, Bug, Ice and Flying) would be 8x weak to that type when affected by Forest's Curse.
Hope it helped!