It is a compound adjective, meaning it describes a noun together.
Here's why:
* Compound Adjectives: When two words work together to describe a noun, they are often hyphenated.
* "Pop-up" as a Modifier: In phrases like "pop-up store" or "pop-up ad," "pop-up" acts as a single descriptive unit.
Example:
* Incorrect: The pop up store was popular.
* Correct: The pop-up store was popular.