A Copy Merged creates a snapshot of your image, including all layer blending modes, adjustment layers, and layer masks, if you have any.
To create a Copy Merged, you first of all need to select your entire image by going to Select > All in the top menu bar. Alternatively, you can also press CMD + A on a Mac or CTRL + A on a PC.
Next, go to Edit > Copy Merged in the top menu bar. This creates a snapshot of your overall image.
Now you can paste the Copy Merged onto a new layer on top of your Layers Panel by selecting Edit > Paste. Likewise, you can also press CMD + V (Mac) / CTRL + V (Win) on your keyboard.
If you like to do some finger acrobatics, you can press and hold Shift, Option and CMD (Mac) / Shift, Alt and CTRL (Win) and press the letter E. This will create a Copy Merged and automatically paste it on top of your layers.
When switching off the visibility of your top layer, you’ll see that the original image looks exactly the same as the Copy Merged that you’ve just created.
Now you can go on making edits on the copy, without harming the original layers, which remain intact below the copy.