Hey there kiddo! Do you know what a plane is? It's like a flat surface, kinda like the top of your desk or the ground you're standing on.
Now let's talk about light. Do you know that light travels in straight lines? Yup, just like when you draw a line with your crayon.
When a beam of light hits a flat surface, it gets reflected (bounced back). The "plane of incidence" is like the imaginary flat surface where the light beam hits. It's like the surface of a mirror that the light hits and bounces off of.
Think of it like playing pool. The ball is like the beam of light and the pool table is the surface it hits. The imaginary line where the ball hits the table is the "plane of incidence".
Sometimes, the plane of incidence can be at an angle instead of being straight up and down like the pool table. That means the light gets reflected at a different angle too! It's like when you throw a ball at an angle, it bounces off in a different direction.
So that's pretty much it. The plane of incidence is just the imaginary flat surface where the beam of light hits and gets reflected off of. Easy, right?