When you first start to build levels in Unity, primitive and placeholder objects can help you "block out" your level or use as player characters. Blocking out is simply just placing objects within the environment where the final version (even if it's not yet created) is intended to go.
These objects are a cube, sphere, plane, capsule, cylinder, and quad, and they are displayed here:
In the image above, you can see an example of each type of primitive/placeholder objects within Unity, along with their pivot point
(arrows located in the center point to each of the 3 axis - x, y, and z).
(arrows located in the center point to each of the 3 axis - x, y, and z).
You can add them by navigating through the top bar GameObject / 3D Object. Since these are 3D objects, we won't be discussing them in this book, but you can read more about these objects by visiting the link in the official documentation at https://docs.unity3d.com/Manual/PrimitiveObjects.html or by scanning this QR code:
