I think it means that you are supposed to create a PSA about something you’re passionate about. For example, if you’re passionate about the environment, you might choose to do a PSA about littering.
The word <em>over </em>is used as an adverb in this sentence.
If the sentence said - There will be clouds over most of the country, then it would be a preposition.
Football is used to describe player as a noun! :)