Flatten, Merge All, Back & Fwd
The Flatten, Merge All, Back & Fwd buttons in Photoshop are basic tools that are designed to help your workflow move a little quicker.
The Flatten button simply takes all of the layers that you have and flattens them all into one single layer, applying any adjustment that you’ve made into the single layer. I recommend saving your image with all it’s layers so you can do further adjustments, flattening your images when you’re ready to sharpen, resize and send off to the printer or put your images on the internet.
The Merge All button creates a merged copy of all of your active layers and places it on top of your layer stack. This is the same as pressing SHIFT+CMD+OPT+E (Mac) or SHIFT+CTRL+ALT+E (pc). Typically you would do this if you want to take the image into an external plugin, or use the filters in Photoshop.
The Back and Fwd button are your Undo and Redo buttons.
The video below will show how to use these tools in more detail.