Dear child,
Imagine you have a cube-shaped toy with six faces. A face diagonal is like a line that goes from one corner of a face to the opposite corner of the opposite face, passing through the inside of the cube.
So if you were to draw a line from one corner on the top face of the cube to the opposite corner on the bottom face through the middle of the cube, you would have a face diagonal.
A fun way to remember this is to pretend you have a tunnel going through the cube and you want to walk from one corner on a face to the opposite corner on the opposite face. That's the path of the face diagonal.
I hope this makes sense!