A layer is extremely important in After Effects as this is what helps us build a composition. We can also use layers to show a video on a TV screen for example. This is done by using masking. See below the example that I created during lesson. This was done by masking the video, so the video shows on the TV. I also added opacity on the mask layer to show a reflection and I used a slight feathering to give the video a more 3D look.