This option is the “undo” button. It will remove any mirroring or cloning (the next two options) you’ve done.
Mirror right/Mirror left
These options will take your layer and mirror it. If you choose to mirror left, a mirror image of the left side will appear on the right in the 3D preview. If you choose to mirror right, a mirror image of the right side will appear on the left of the 3D preview. It should produce a symmetrical preview.