It's not very effective when the type resists it, and usually in games they show it's not very effective with a sound or indication after the move hits. When it's regularly effective, it means the Pokémon doesn't resist the move, and neither is hit super-effectively. Or, the Pokémon has two types, and one of those types are hit super-effectively by a type, but the other type of the Pokémon resists that type. A Pokemon is hit super-effectively when the move it is hit by gets 2x of the normal power.