What is a prepositional phrase (murakkab jārrī)?

A حَرْف جَرّ (harf jarr) never stands alone. It takes a noun after it, and that noun is the اِسْم مَجْرُور (ism majrūr).

A harf jarr with the noun after it: the noun is always majrūr, and the pair together forms a shibh al-jumla.

In فِي الْبَيْتِ (fī al-bayti), “in the house”, فِي is the harf jarr and الْبَيْتِ is the ism majrūr, showing its kasra.

Together the two make a شِبْه الْجُمْلَة (shibh al-jumla) — a quasi-sentence. It is not a sentence, but it attaches to something in the sentence, and that attachment is called تَعَلُّق (ta'alluq).

Like most Arabic grammar terms, جَرّ (jarr) was taken from everyday speech and then given one precise technical sense.

In everyday Arabic it means pulling or dragging something along.

جَرَّ الْحَبْلَ (jarra al-habla) — “he pulled the rope”

In Arabic grammar it names the case a preposition pulls the following noun into.