Propagator

Okay, so let's start with a story.

Imagine you have a little toy boat that you want to move from one end of the bathtub to the other. You pick up the boat and set it down in the water at one end, and it slowly drifts to the other end over time.

Now, what if you wanted to figure out exactly where the boat would be at any given time between the two ends of the tub? That's where a propagator comes in!

A propagator is like a magic tool that helps you figure out how something moves over time- like your toy boat moving through the water. It takes into account things like the starting point, the speed of the boat, and any other factors that might affect its movement.

So, using a propagator, you could figure out exactly where your toy boat would be at any specific time in between the two tub ends. And that's pretty useful for all sorts of things- like predicting the movement of particles in physics, or the spread of a disease in epidemiology.

In short, a propagator is a tool that helps you figure out how something moves from one point to another over time. It's like a magic wand for predicting the motion of things!