As you're going around saving London from Blighters, you'll come across sections that need freed that are marked with a ball and chain. Those are the Liberation missions. Every one is different, with a different amount of kids you have to save, but they all share the common "Don't trigger the Alarm Bell" side option.
Note: You are NOT REQUIRED to not have the bell rung for this achievement, it's just more of a perfectionist thing.
Complete five of these throughout the boroughs and the achievement is yours.