The layers feature is simply a way of displaying the objects (photos, text, graphics) you’ve placed in a design so you can easily manipulate. The layers pane is a visual display of all the objects in your image.
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The layers are displayed on the left pane of the document editor, below the toolbar where you find the text, image and shape buttons.

Every time an object (i.e text, image, shape) is added to the document, a new layer will be created for that object and it can be seen on the left under the layers pane.

Here is an example on how layers can help you in designing your document:
Let's say you want a rectangle to go behind a text, you can find the layered named “rectangle” and drag it below the text layer you want to show in front or vice versa