Harf: a particle
حَرْفharf - particle
Harf is a particle whose meaning is completed through another word. Learn amil and ghayr amil particles.
Simple explanation
حَرْف (harf) is a word whose meaning is completed through another word. Particles are small but powerful: they connect words, change roles, and build many common Quranic phrases.
A useful early classification is whether a harf is عَامِل (amil) or غَيْر عَامِل (ghayr 'amil).