Okay, let's imagine that you have a big jar of candy. Sometimes, if you are doing really well in school or helping with extra chores at home, your parents might surprise you with an extra piece of candy from the jar. That is like a special dividend.
In grown-up world, it's kind of the same thing. When you buy something called a stock in a company, that means you own a little piece of that company. When the company does really well and makes a lot of extra money, they might decide to give some of that extra money to their owners (people like you who own the stock). This is called a dividend.
Now, a special dividend is just like an extra, extra piece of candy from your parents. It's money that the company gives to its owners (like you) on top of the regular dividend. So, instead of just getting a little bit of money every quarter, you might get a big chunk of money all at once if the company is doing really well. It's kind of like a "thank you" gift for being a good owner of the company's stock.