Have you ever noticed how things look different when the lights are on or off? When the lights are on, things are bright and you can see everything clearly. But when the lights are off, things look dark and spooky. This is because the light affects how we see everything around us.
Unified lighting and shadowing means that everything in a picture or video is affected by the same lighting and shadows. This makes everything look more realistic and like it belongs in the same place. It's like turning on the lights in a room, everything is visible and looks like it belongs together.
Think about a drawing of a person that you did. If you want to make it look realistic, you have to consider the light source. Where is the light coming from? What areas of the person's face or body would be in shadow because of the light? This is what unified lighting and shadowing is about. It's making sure that everything in the picture looks like it was in the same place with the same lighting and shadows.