Prepositional phrases usually tell us about when or where.
Let's think about an apple. It may be on, in, under or beside the table. Or think about the apple's movement like if the apple flies "through the air," "toward the wall" or "away from the tree."
Prepositional phrases describe which one such as the book is "on the desk" or how, when or where. For example, "William talked to Tessa in the car." The phrase "in the car" describes where the talking took place, so it is a prepositional phrase.