i've met a few of friends in the town centre. → in this sentence, "of" is used after the quantifier "a few" to indicate a subset or selection from a larger group of friends.
a few of the streets in the town centre have street entertainers. → here, "of" is used after "a few" to specify a subset of streets within the town centre that have street entertainers.
most of these students live in my neighbourhood. → similarly, "of" is used after "most" to indicate a subset of students from the larger group mentioned.