So you're on the path to take the next big step in your Photoshop skill knowledge and want to explore the concept of making use of layers and layer masks!
By now you should feel comfortable with opening up an image into Adobe Photoshop CC, making a few adjustments, perhaps with the help of a few selection tools, using filters such as blur and sharpening filters, the History Panel and understanding the saving process as well.
By now you should be aware that these adjustments you are making are rather destructive and rather limiting as well. Whereby the concept of using layers, adjustment layers and layer masks will not only reduce the destructiveness of your workflow (not being able to correct or undo an adjustment you did right at the beginning of your project without losing everything you did after that point) but will also ensure you unlock an entirely different mindset of how digital image manipulation works.
Here is a fantastic beginner guide on how to use layers within Adobe Photoshop CC:
Take a look at the video below by the YouTube channel: Pixel & Bracket on Layers For Beginners - Photoshop CC Tutorial...