Additive number theory is like playing a game with numbers.
Let's say you have a big number like 10. Additive number theory is like trying to figure out how many different ways you can make 10 by adding together smaller numbers. For example, you can make 10 by adding 4 and 6, or 5 and 5, or 2 and 8.
But the really tricky part comes when you try to figure out if there's a pattern to how these combinations work for all numbers, not just 10. This is what additive number theory is all about: trying to find out how to add up any number using smaller numbers.
Some smart people have figured out some rules that seem to work for many cases. One rule is called the "Goldbach conjecture," which says that any even number can be made by adding two prime numbers (numbers that can only be divided by 1 and themselves, like 2, 3, and 5).
But there's still so much we don't know about additive number theory. It's like a puzzle waiting to be solved, and people are still working on it today.