Layers are used to indicate some functionality
across many different game objects in Unity.
For instance, layers can be used to indicate
which object should be drawn in the
scene view, should ignore raycasts,
or are invisible to the camera.
Here we have a scene with 2 orbs in it.
We can see that both orbs are on the layer Default.
If we wanted to change the layer of 1 of the orbs
we could click the Layer drop-down in the inspector
and select a new layer.
We can see that this orb is now on the
Ignore Raycast layer
and will not be hit by any raycasts that pass through it.
We also have the ability to define our own layers.
We can do so using the Tag And Layer Manager window.
To access the tag and layer manager window
either select Add Layer from the Layers drop-down
or click Edit - Project Settings - Tags And Layers.
Here we can see all of the built-in layers
as well as add our own.
Let's create an Ignored By Camera layer.
We can place our other sphere
on this newly created layer
by selecting it and choosing the New Ignored By Camera layer
from the Layers drop-down.
This sphere is now on the
Ignored By Camera layer.
The layer has no inherent functionality however
so let's modify our camera to ignore it.
In the Culling Mask property of a camera
we can select which layers are seen by the camera
and which are not.
By clicking the drop-down and
unselecting the Ignored By Camera layer
all objects on that layer will no longer
be visible to that camera.
Let's run our scene.
We can see that only a single orb appears.
The other orb, the one on the Ignored By Camera Layer
is not rendered to the screen.
We can also use layers to determine
what appears in the scene view.
By clicking on the Scene Layers drop-down
at the top right of the Unity editor
we can select and deselect layers.
Doing so will cause all objects
on those layers to disappear.