What is a small horizontal line used to set off parts of sentences or create compounds?

The small horizontal line you're describing is called a hyphen.

Here are some examples of how it's used:

* Compound words: "check-in" "well-being" "mother-in-law"

* Compound adjectives: "state-of-the-art" "user-friendly"

* Number ranges: "pages 25-30" "1990-1995"

* To avoid ambiguity: "re-create" (to create again) vs. "recreate" (to create)

Let me know if you'd like more examples or have any other questions!