Arooz is the classical science of poetic meter — the study of the rhythmic patterns of long and short syllables that make a verse mauzun, in-meter. It was systematized in the 8th century by the Arab philologist Khalil ibn Ahmad al-Farahidi for Arabic poetry. Persian poets adopted and adapted it; Urdu inherited the system from Persian in the 17th and 18th centuries and made it its own.
Urdu meter is quantitative, not stress-based like English. What matters is whether each syllable is long (a vowel followed by a consonant, or a long vowel such as ا / و / ی) or short (a single consonant with a brief vowel sound). A bahr is just a fixed pattern of longs and shorts. To say a line is in a particular bahr is to say its syllables — read in order — match that pattern exactly.
This tool exists to do that scanning for you in seconds. But knowing why a verse works is what turns a reader into a poet.