Okay kiddo, so let's say you have a lemonade stand. You sell lemonade for $1 per cup, and it costs you 50 cents to make each cup of lemonade (that's the cost of the lemons, sugar, cups, etc.).
Now, your business margin is the money you make after you've paid for everything it takes to make the lemonade. So if you sell 10 cups of lemonade in one day, you've made $10 in revenue, but you've also spent $5 on the ingredients. That means your business margin for the day is $5.
That might not seem like a lot, but it's important because it tells you how much money you're making on each sale, and whether your business is making a profit or not. To make your lemonade stand more profitable, you could try to increase the price of each cup (but be careful, because if you make it too expensive, nobody will want to buy it!), or find ways to lower the cost of the ingredients.
So that's business margin, in a nutshell - it's the money you make after you've paid for everything you need to sell your product.