Checking off a grocery list item doesn't permanently remove it from the grocery list. Rather, the completed items are moved out of sight into a 'completed items' area (on iOS and Android).
Show xx completed items to show the completed items. You can bring them back to the grocery list by unchecking them, or you can view their extra ingredient details by clicking on the right hand side of the item.
Hide the completed items by tapping
Hide xx completed items.
If you've completed your entire grocery list and want to view all of the completed items or modify the list, tap the
Show xx completed items.
On the web app, completed items are shown in a category called 'Got'em', located at the bottom of the list.