Alright kiddo, so let's say you're outside on a sunny day and you try to look through a really thick cloud. You might notice that it's harder to see things behind the cloud. This is kind of like what we call "optical depth".
Basically, optical depth is a measure of how much light gets blocked as it travels through something. When light travels through something, like a cloud or a gas, it can get absorbed, scattered or reflected by the particles in that material. The more particles there are, and the bigger they are, the more likely it is that some of the light will get blocked.
Scientists use something called "optical depth" to measure how much light gets blocked as it travels through a material. Think of it like measuring how many toys you can see through a stack of blocks - the more blocks you add, the harder it is to see what's on the other side.
The optical depth of a material is usually represented by the letter "τ" (pronounced "tau"). If something has a high optical depth, it means that a lot of light is getting blocked as it goes through that material. If something has a low optical depth, it means that not much light is getting blocked.
So, optical depth is just a way of measuring how much light gets blocked as it travels through something. It can be really useful for scientists who study things like the atmosphere, because it lets them understand how much light is getting through and how much is being absorbed or scattered by the air or clouds.