Hiya Rob, welcome to the PSG.
Could you be a bit more explicit about your difficulty.
Basically, if you can think of layers as individual "slides" which are stacked one on top of the other to make up a Photoshop document.
A layer can be:
A image filling the whole slide.
A cut out with transparent areas.
Text.
A pattern, texture, painted or drawn slide.
The individual layers appear in the Layers palette and are "viewed" as if looking down from the top.
The layers can be restacked or re-ordered.
Look in the Help menu for further info. Read the User Guide and try some of the beginner walk-throughs.
Photoshop is vast, it will take a bit of study to get the best from it.
Hope this gets you started.
Cheers Al.