Prepositional phrases add meaning to verbs and nouns in sentences.  In this way they function in an adjective or adverb-like manner.
When they act like adverbs they are often giving answers to the questions how, when, or where?

John is tired from this afternoon’s exam. (why)
Before the semester starts, most university students will receive their timetable on line. (when)
The terrorists planned to meet at the south end of Chatham Road. (where)
The protestors refused to leave until the police arrived (for how long)