Super Effective means double damage, or x2. You do twice as much damage.
Super Effectiveness:252 SpA Life Orb Greninja Hidden Power Water vs. 252 HP / 4 SpD Heatran: 205-244 (53.1 - 63.2%) -- guaranteed 2HKO
Normal Effectivness:252 SpA Life Orb Greninja Hidden Power Dark vs. 252 HP / 4 SpD Heatran: 103-122 (26.6 - 31.6%) -- guaranteed 4HKO
26.6 - 31.6% is for normal and 53.1 - 63.2% is for super effective. See how super effective is 2x as much damage as normal? It does twice as much. I am doing not very effective for your convenience,
Not Very Effective:252 SpA Life Orb Protean Greninja Hidden Power Psychic vs. 252 HP / 4 SpD Heatran: 51-61 (13.2 - 15.8%) -- possible 7HKO
2x less chance to KO than Normal Effectivness, except for that 7HKO, but see? It does half damage.
Hope I helped!