Okay kiddo, so you know how grown-ups have to save money for when they are old and can't work anymore? They do this by putting some of their money into something called a retirement plan.
Plan 55-a is a special retirement plan that some people who work for the government can use. It's like a piggy bank where they can put their money and it will grow bigger over time.
When they get old and retire, the government will give them that money back so they can use it to buy food, pay for a house, and other things they need.
Plan 55-a is a good way for people who work for the government to save their money for the future when they can't work anymore.