Layers allow you to separate behaviors into multiple interchangeable Stacks. The idea is that the lower Layers are more stable and host Stacks that deal with static controller mappings (group, master faders for instance), choosing between or merging behaviors generated by the higher layers, etc.
The current layer is indicated via the Layer Indicator, shown below.
The result of the Base (lowest) layer is what eventually goes out into the Fixture. If you want to use generated Aspects from other layers, you have to employ the Source.Layer Block in your Stack.
Go Up a Layer
Press PageUp or Click the Up Arrow in the Layer Indicator
Go Down a Layer
Press PageDown or Click the Down Arrow in the Layer Indicator