Harf jarr: the connecting particle
حَرْف جَرّharf jarr - particle of jarr
Harf jarr is a particle that makes the following noun majrur and forms murakkab jarri.
Simple explanation
حَرْف جَرّ (harf jarr) connects to the noun after it and causes that noun to become مَجْرُور (majrur).
Together with the following noun, it forms a very common Quranic phrase pattern.