I believe a cart is 'abandoned' after a certain amount of time of inactivity. Carts are stored in the database and in the user's cookies (usually by default, this can be changed).
If you add items to your cart, then you just leave the site, it keeps the cart for a certain period of time before it cleans up old carts (it can be changed in the Settings)
Anyway, when a cart is finally deemed "abandoned" the trigger is for you to use for things like sending the customer a "Hey, you left your cart, would you like more information on the items to help you make a buying decision?" or something marketing-like.
https://docs.kentico.com/k10/e-commerce-features/customizing-and-developing-your-store/shopping-cart-related-customizing/customizing-marking-shopping-carts-as-abandoned <- More on how to use it and modify the behavior.