A sunset is what occurs as the sun passes below the horizon in the transition from day to night. Because of the way light is refracted from the low angle, they often appear a brilliant red or orange colour. Note that 'sun set' is not the same thing - 'sunset' is a noun, whereas 'sun set' is a noun and a verb - telling us what the sun did (it set).
E.g.
Here are some more examples using the noun.
It is difficult to describe the beauty of a sunset.
There is a beautiful sunset happening right now.
The adventurous man trotted his horse off to the west into the sunset.
The light at sunset seemed supernal, bathing the meadows and mountains in its glow.
The Grand Canyon at sunset is truly a sight to behold.
I was struck by the pulchritude of the sunset over the ocean.
No, that's a phrase, not a sentence. A complete sentence which includes that phrase might be, I like watching the brilliant sunset over the desert.
A gentle caress from his wife restored faith in him.
A geographer could tell you what kind of rock is in that formation; all I can do is tell you I love the shadow it casts at sunset.
The lovely sunset shined across the ocean blue water the sight was very gorgeous and scenic
"By God, that was the most incredible sunset I've ever seen."
The sun is setting.