An abandoned checkout is exactly that - a cart that the customer did not follow through to completion. The truth is that the majority of carts that are created are not completed as orders. This is a fact in any product market, as far as eCommerce is concerned. This is why abandoned checkout emails (which are part of Shopify and also available via apps) are a valuable resource for convincing customers to complete their transactions.
Customers fail to complete transactions for a number of reasons. Sometimes they just change their minds, sometimes they don't like the shipping fees or price, or they have decided to price shop and look for a similar product elsewhere. Engaging with your own customers is the only way to find out for sure why they may not be completing their orders.
An abandoned checkout is a situation where a customer has added items to their cart, provides some contact information, but does not complete the checkout. Basically they bailed at the last step, entering in payment information. Shopify knows when this happens and stores this information. You can read more about it here: https://docs.shopify.com/manual/your-store/orders/abandoned-orders
If you would like help recovering those users, you could also try our app called TargetGenius. It allows for you to send browser push notifications to users 24 hours after they've abandoned their checkout. No work on your end! Find us in the Shopify app store here: https://apps.shopify.com/targetgenius. Let's wrangle those abandoned carts together!