"Is" is a verb, and verbs usually require a subject and an object to complete their meaning. For example:
* The cat is *sleeping*.
* The sky is *blue*.
In these cases, "is" acts as a linking verb, connecting the subject (cat/sky) to the object (sleeping/blue).
However, there are some informal or creative contexts where a sentence might seemingly end with "is," but there's usually an implied understanding of the missing object:
* "What is that?" (implied object: "thing")
* "Where is it?" (implied object: "located")
Let me know if you'd like to explore other grammatical concepts!