White Reveals, Black Conceals..... You have it the wrong way round.
When you create a layer mask, generally the mask is white. So paint black on the mask to hide portions of the layer.
You can select the layer mask and fill it with black then you use the brush tool with the foreground set to white to reveal parts of an image.
Depends what your particular task is. Once mastered your editing will improve a lot.
Sorry I was the wrong way round, but what I was really getting at was that the OP appeared to be clicking on the layer thumbnail rather than the mask icon.
sorry Roy. it ooks like he was following a video tutorial.


