Parent Theme (Booklium) is the main theme for all the functionalities and features of the theme will have. Theme can work standalone, it’s not compulsory to have a child theme.
Child theme (Booklium child) is usually used to add extra styles, functions or features without editing the main Theme or Parent Theme. This is a safe practice to add extra features. We recommend installing a child heme, in order to make the made changes (to the code) update proof. Child Theme required a parent Theme to work, it’s can’t work without a parent theme.