What is ‘arūḍ?
Classical Arabic prosody describes lines as patterns of short and long syllables organised into feet (tafʿīla) and metres (buḥūr).
Al-Khalīl after practice
In the 8th century al-Khalīl ibn Aḥmad mapped recurring patterns from existing poetry. Theory followed practice, with recognised variations.
A small exercise
Take a short line you know, read it slowly, and mark length. Compare a metrical line with a prose paraphrase of the same idea.