This is an analogy for our mind vs what arises and ceases within our minds.

Sunrise and sunset within empty space – is like our thoughts and emotions (i.e., mental activity) rising and setting within our minds. These thoughts are NOT the mind itself but merely things which appear and disappear within our minds.

Thoughts are just thoughts and are not self. Thoughts are not who you really are – they are just mental energies that rise and cease within our minds but are not the mind itself:

The Mind (that which is aware of the thoughts) is the host – it is always present

Thoughts, emotions are mere guests – they come and go.

Another analogy is reflections in the mirror are mere appearances (not reality) that appear and disappear within the mirror. Yet these reflections are not the mirror itself. Whether something beautiful appears in the mirror or whether something ugly appears in the mirror – the mirror itself is neither beautiful nor ugly. The mirror remains unchanged, untouched in the slightest by whatever appears within it. The mirror is the substance within which all the illusory reflections appear but the reflections are not reality.

So, Master Huang Po said:

3. Mind is like the void in which there is no confusion or evil, as when the sun wheels through it shining upon the four corners of the world.

For, when the sun rises and illuminates the whole earth, the void gains not in brilliance;

and, when the sun sets, the void does not darken.

The phenomena of light and dark alternate with each other, but the nature of the void remains unchanged. So it is with the Mind of the Buddha and of sentient beings.

If you look upon:

the Buddha as presenting a pure, bright or Enlightened appearance,

or upon sentient beings as presenting a foul, dark or mortal-seeming appearance,

these conceptions resulting from attachment to form will keep you from supreme knowledge, even after the passing of as many aeons as there are sands in the Ganges.

There is only the One Mind and not a particle of anything else on which to lay hold, for this Mind is the Buddha. If you students of the Way do not awake to this Mind substance, you will overlay Mind with conceptual thought, you will seek the Buddha outside yourselves, and you will remain attached to forms, pious practices and so on, all of which are harmful and not at all the way to supreme knowledge.

– The Zen Teaching of Huang Po, John Blofeld translation