There are two types of layers.
An adjustment layer which applies an adjustment to the layer below. Sometimes the subtlety of an adjustment layer is difficult to see in a demonstration.
The other layer adjustment is one where there are actual pixels and that is the easiest way to demonstrate the effects of a layer and layer mask.
First open two images of equal size and orientation in PS. Pick one of the images, it matters not which :-). Select All and Copy (cmd A, followed by cmd C on a Mac). Move to the second image and Paste (cmd V on a Mac)
In order to demonstrate the layer and layer mask we must add a Mask. It is the washing machine logo at the bottom of the layers palette. Make sure the focus is on the top layer (white line around the image in the layers palette.) when you click it.
Now there should be a white line around the layer mask.
Pick a black brush and draw on the image.
You will see that a black brush reveals the layer below and a white brush will hide it again.