^ multis are multi-syllable rhymes... allow me to explain

They back slide back to church, call a MINISTER'S BLUFF
They'd rather remain unenlightened than LISTEN TO 'BUS

(from Genabis by Canibus)

the words in caps all rhyme at the same time... rap that bar back to your self, and you'll find that

ministers bluff

and

listen to bus

rhyme with the same amount of syllables completely... thats a 4 syllable rhyme...

so basically, a multi is a series of rhyming syllables right next to each other.. anyway thats what i think it is... the reason its done is to showcase lyrical strength, plus it sounds better since theres more actual rhyming.. hope that helped