A throng of people is a large group of people: usually a large group that's moving around and making noise. And, when people or things throng, they push themselves tightly together into a big, moving, noisy group.
You might join a throng of students in a crowded hallway, or join a throng of protestors for a noisy rally.
When the doors open at a concert or a sporting event, fans will throng into the stadium or the arena. When stores are offering deep discounts on Black Friday, shoppers will line up before the store opens, then throng into the store as the doors open. And during rush hour, commuters will throng into an open car on a subway or a train.