Let's break it down:
* For: This is a preposition, introducing the prepositional phrase.
* Moment: This is a noun.
* For a moment: This entire phrase functions as an adverb, modifying a verb and indicating the duration of an action.
Example:
"He stopped for a moment to catch his breath."
In this example, "for a moment" tells us how long he stopped, making it an adverb modifying the verb "stopped."