If a subordinate clause does not modify anything but it functions as the subject, direct or indirect object, predicate nominative, or an object of a preposition, then it is a(n) clause. elliptical
adverb
noun
adjective

It says in the text that millions or people had come from different parts of the country to go into Washington, D.C. to protest. So congregate means to gather around in a mass.

So the context clues would be “meet on the National Mall” “millions of people” “their voices heard”.

hope this helps