Everyone has an idea in her head when the word layers is mentioned - it might just be that at this time of the year in the northern hemisphere it relates to how many loads of clothes one can put on! Think of a lasagne - layers of pasta/meat/Bolognese sauce/cheese and visualise the colours.
A sandwich and what is inside. As a child I loved club sandwiches and enjoyed the different colours inside the bread. In fashion the layering concept comes and goes but Ian has only just discovered about the trendiness of 'layering'. For me, apart from what makes up the layers - the goodies - are the colours in the layers and how you can expose some or all in places.