Store Cart feature in EShop allows the site to still hold the cart data of customers if they have already added the products into the cart but have still not checkout yet. So after adding products into the cart, if the customers go away or logout from the site, then they come back later and login to the site again, the cart data is still there so they can continue to browsing the store easily.
To enable the Store Cart feature, you need to go to EShop -> System -> Configuration -> Option tab, at "Store Cart", choose Yes. Once Store Cart is enabled, you can also configure the "Store Cart Schedule" to decide the time frame before the customer's cart data is removed.
In the order to have Store Cart feature works properly, you need also to go to Plugins Manager in Joomla at System -> Plugins to enable the 2 following plugins: