I could’ve sworn I replied to this post… scratches head
Photoshop is a rather deep program, so I’d recommend you check out some tutorials (whether built-in or online) to really learn about the program.
anyway, the concept behind layers (whether in Photoshop, Freehand, etc) is simple - think of a basic ham & cheese sandwich.
The bottom (1st) layer is your bread, the 2nd layer is your cheese, 3rd layer your ham, 4th layer the top slice of bread.
They have an order (bread-cheese-ham-bread). You can change the order around (bread-ham-bread-cheese), add layers (bread-ham-bread-cheese-bread), and delete layers (bread-ham-cheese-bread). You can also hide layers but that’s not applicable to the sandwich example.
What this means is that if you put something on its own layer, you can edit it by itself (run filters, change its hue/saturation/opacity/etc) and not affect the rest of the pic. In sandwich terms, that’d be taking out the slice of ham & grilling it with BBQ sauce. The ham now tastes of BBQ sauce but the rest of the sandwich is unaffected.
Transparency on layers
Assuming we’re talking about the kind of Swiss cheese with the holes - when you slap that piece of cheese on the bread, you can still see the bread through the holes, right? Similarly any layer with parts that don’t have anything on them (transparent areas) will reveal what is in the layer(s) below it. Your layers start out empty, so it’s intially all transparent.
that’s about the basics of layers. good luck.
if you need more help, there’s always Google.