Holography
Holography is a way to record light so a flat surface shows a picture that looks three-dimensional.
Think of it this way
A stone drops into a still pond and sends out ripples. The ripples cross each other and form a pattern on the surface. A flat sheet can record that pattern as thin lines. Light shining on those lines brings back the full shape with depth.
Explaining holography by grade level
When you scratch lines into a sheet of plastic with a compass, you make tiny grooves. Light bounces off those grooves in a special pattern. Your eyes see that pattern and it looks like a shape floating above the plastic. The picture changes when you tilt the sheet.