Working with layers

When working with layers in SketchUp 2019, there is a pencil icon at the right hand side of Layer0. What is that icon for? How does it work’

That pencil changes the active layer.
Basically you should never need to touch it.
Read up on layers here, and note that the pencil is the radio button on the left that has been moved in 2019.
https://help.sketchup.com/en/sketchup/controlling-visibility-layers

Thankyou very much! It was very helpful, specially the link where it is widely explained.