When organizing a closet, people usually group clothing by type. You will see a pile with t-shirts, a pile with pants, a socks drawer. And now, whenever you want to pick something, you go to the group that should have it, see all of the items in there and pick the right one.
Now, if we wanted to, we could add more detail to the arrangement. Instead of having a socks drawer we can have a green socks drawer, another for red socks, another for white socks, and so on. As long as we know in which drawer the sock we are looking for is, we can look it up faster, since we will have less incorrect items in the drawer.
Socks made by Freepik from www.flaticon.com. Licensed under CC BY 3.0. Colored by Sebastian Waisbrot.
A Hash Map groups things by characteristics. If we have enough groups, we can add and look up any item really fast.