In general, a preposition is anything a cat can do. For example, in, on, under, next to, beside, etc. Prepositional phrases answer the question Where?, like this,
EX: Max went to the store.
"to the store" is a prepositional phrase. "to" heads the phrase. It's a preposition. "to the store" tells us where Max went.
Prepositional phrases also answer the question When?, like this,
EX: I watched TV during the exam.
"during" is a preposition and it heads the prepositional phrase "during the exam".
Note that, prepositional phrases have a form and a function. They are made up of a preposition and a noun phrase. They also have function: they function as adverbs. They can tell us when and where something happens.
All the best.