Parish Online has lots of different layers of information which are organised in Map Layers. Parish Online is a GIS (Geographic Information System) and the best way to understand this concept is by thinking of an Overhead Projector (you probably have to be over 30 to understand this reference...).
Imagine a Map Layer is a layer of transparent acetate placed on an overhead projector. Whatever is written or drawn on that acetate is displayed on the wall. If you build up a collection of these acetate 'layers' then you can put multiple layers on top of each other in any order to show different things.
Now imagine that the wall you're projecting onto can have different wallpaper. In our analogy, the wallpaper is your base map. This can be either Ordnance Survey mapping or Aerial Photography. It gives your layers of information context (i.e. your drawing is placed on top of the base map).
Why are Map Layers important?
Organising the information you draw in Parish Online into separate 'layers of acetate' is important in giving your data structure and styling. If your street lights are in a different layer to your grit bins then you can switch them both on at the same time, but you can remove just one of them if it's not needed. It gives you the flexibility to create an infinite number of maps.
Choosing Map Layers
You should aim to always have one base map switched on. Base maps are in the orange collections (collapsible folders).
You don't need to have any Map Layers switched on. When you want to view a certain layer, simply find it and switch it on. You can switch it off at any time.