A Human megaphone, also known as a Human microphone or People's Mic is used when amplified sound is not feasible and there are too many people to easily hear the speaker.
The speaker says just a few words at a time, then pauses as the people who can hear what she said loudly repeat the phrase so people further from the speaker know what was said. For very large crowds, multiple echos might be necessary.
Effective use of the People's Mic requires speakers be concise and avoid the temptation to say phrases of more than a few words at a time.
A "Mic Check" is how a speaker gets attention to the People's Mic and ensures everyone can hear: Everyone repeats the speaker's "Mic Check" until the speaker can be heard through the whole crowd.