A negative number is like when you owe somebody money, but with math. Just like how you can have 5 cookies or no cookies, you can also have -5 cookies, which means you owe somebody else 5 cookies.
When we count, we usually start at 0, then go up by 1 for each thing we add. But with negative numbers, we start at 0 again and go down by 1 for each thing we owe. So if you owe somebody 1 cookie, that's -1.
Negative numbers can also help us with things like directions. If you're walking to the left of a map, that could be -3 miles from the start, because you're going backwards from where you began.
Overall, negative numbers are just a way to show when you owe somebody, or when you need to go backwards from where you started.