Okay, imagine you have a large bucket of water and you want to see how the water moves and spreads out.
Now, let's say you pour a little bit of mud into the bucket. The mud will spread out over time and eventually mix in with the water. The way that the mud spreads out is kind of like a puzzle, and we can try to figure out how the puzzle pieces fit together using something called the convection-diffusion equation.
The equation is trying to tell us how the water currents in the bucket are moving the mud around (that's the convection part) and how the mud is spreading out on its own (that's the diffusion part).
But the equation is really tough to solve by hand, especially if we want to study how the mud spreads out over a long period of time, or if we have a really big bucket of water. That's where numerical solution comes in.
Instead of trying to solve the equation by hand, we use a computer program to break it down into lots and lots of tiny steps. Each little step tells us how the mud is spreading out just a tiny bit more. We keep doing this over and over, adding up all the little steps, until we have a pretty good idea of how the whole puzzle pieces fit together.
This way of solving the equation is like building a really big puzzle one tiny piece at a time. The computer can do it really fast and accurately, which helps us understand how the mud is moving around in the bucket!