Okay, let's imagine you are at the beach and you have a sandcastle. Now, you want to put some water on the sandcastle to make it wet. You take a bucket of water and pour it on top of your sandcastle. Suddenly, the water starts to move around the sandcastle and some of it might even get absorbed by the sand.
Now imagine that the sandcastle is made up of tiny particles (called ions) and the water is what we call an electrolyte solution (which just means it has some salt or other particles dissolved in it). When the water touches the sandcastle, it causes the ions to move around and they can even be absorbed by the water. This movement of ions is what creates an electric charge around the sandcastle.
Okay, now let's make it even simpler. Imagine you have a bunch of toys (the ions) and a big sandbox (the electrolyte solution). If you just take the toys and dump them into the sandbox, they will be very crowded and might even bump into each other. But if you spread them out evenly, there will be more space for each toy and they won't bump into each other as much. This is what we call a "diffuse" distribution of the toys.
The Poisson-Boltzmann equation (PBE) is a way to calculate this diffuse distribution of ions in an electrolyte solution. It takes into account the electric charge of each ion, the concentration of each ion in the solution, and how far apart they are from each other. The PBE helps us understand how ions interact with each other in solution and how they create this diffuse distribution.
So, to summarize, the Poisson-Boltzmann equation is a fancy math equation that helps us understand how electrically charged particles (called ions) move around and interact with each other in a solution (like water with some salt dissolved in it). It helps us understand how these particles create an electric charge around them and how they spread out evenly to create a diffuse distribution.