Oh wow, how to explain a shader :)
A shader is a small program that runs on the GPU to control how things are drawn. It can determine a pixel's color, apply lighting, create effects like water or fire, or even deform geometry, all while rendering the scene.
For Godo you can find examples here: https://godotshaders.com/
Or get a bit of a look on youtube:
Hope this helps you out mate :)