Cartis where your customers’ products are placed as they shop on your store - you could view it as a holding area for customers’ items, where they can adjust quantity and view all items they are looking to buy.
The term 'cart' can also sometimes be called 'shopping cart' or 'basket', but they all essentially mean the same thing.
When the user updates the quantity of an item in the cart, the cart will update automatically.
Cart is often confused with the Checkout.
The Checkout is where payment and delivery details are taken care of and is outside the control of your theme - this is handled by the core Shopify platform.