Well kiddo, usually, when we put one thing together with another thing, we get two things, right? For example, if we have one apple and we get another apple, we have two apples. But sometimes, when we put one thing together with another thing, we get something even more amazing- THREE things!
Let's imagine we have one toy car and another toy car. If we put them together, we can "build" a new toy car that is even cooler than just one car or the other. So now, instead of having just two toy cars, we have three! That's why sometimes people say "one plus one equals three" when they talk about combining things to make something even better.
It's sort of like when your mom or dad makes your favorite sandwich. They start with one piece of bread, and another piece of bread, and then add some cheese and maybe some ham or turkey. When they put it all together, what do you get? A delicious sandwich that is better than just bread, cheese, and meat by themselves!
So remember, sometimes when we add things together, we don't just get two things- we get something even cooler- three things! That's why people say "one plus one equals three."