Functional integral

Imagine that you have a really cool toy called a "function machine." This machine takes in "functions" as inputs and then spits out "numbers" as outputs. Now let's say that you want to figure out how likely it is for this machine to produce a certain output given a certain input.

This is where the "functional integral" comes in. It's like a special formula that helps you calculate this probability. But instead of just plugging in numbers, you have to use special symbols to represent different possible "functions." Kind of like how you use different letters to represent different numbers in an algebra equation.

Once you've set up the formula with all the right symbols, you can start to "integrate" or add up all the different possible outcomes, just like you might add up all the marbles in a jar. This gives you a final number that tells you the probability of the function machine producing that particular output for that particular input.

So basically, a functional integral is a way to use a special formula to figure out the probability of a function machine producing a certain output for a certain input, by adding up all the different possible outcomes.