Okay little one, so human flow means how people move around and travel from one place to another. Just like how water flows in a river, people also flow around cities, towns, and countries.
You know how when we go to the store, we have to walk in a particular direction and follow specific rules, like staying in line? Well, that happens everywhere all the time. People have to make sure they don't block other people's paths, move safely without hurting themselves or others, and follow rules like stopping at stop signs or traffic lights when they drive or walk on the road.
Think about how many people move around in a busy city like New York or London, for example. There are many ways people can travel, like walking, biking, driving, taking the bus or train or even flying! The way people move around can affect the flow of the city, just like how a blockage in a river can affect how water flows.
So people have to be mindful of each other and work together to keep the human flow going smoothly, just like the way we work together to keep our toys organized and put away after we're done playing.