Some Pokemon Have more than one type but they can only have 2.
Some Pokemon are immune to certain types like ghost cannot damage normal and normal cannot damage ghost.
If a Pokemon is weak to a type it is x2 if it has 2 types than it is 4x weak to whatever it is weak too.
For an example Rayquaza has 2 types: Flying and Dragon
Rayquaza is 4x weak to ice instead of 2x because Flying and Dragon are both weak to ice, but Rayquaza is neutral to electric because dragon is not weak to electric so it cancels out and is neutral.