Neutralization
Neutralization is what happens when an acid and a base cancel each other out, creating something milder.
Think of it this way
Vinegar is an acid and baking soda is a base. When you mix them in a bowl, they fizz and react until both are used up. What is left in the bowl is mostly water — milder than either ingredient on its own. The acid and the base canceled each other out.
Explaining neutralization by grade level
When you mix baking soda and vinegar, they fizz and bubble. That fizzing means the two things are changing each other. The sour vinegar and the powdery baking soda work against each other until neither one is strong anymore. What you have left is mostly water with some salt mixed in. The strong stuff became gentle.