A parent theme in WordPress is a theme that is declared parent by a another theme, child theme. This feature in WordPress allows theme designers and developers to take advantage of a larger and robust WordPress themes and make modifications to those themes by creating child themes. A Parent theme passes along all its functionality, features and style to the child theme. The child theme can selectively make changes into the functionality of the parent theme without ever modifying the Parent theme.
The parent/child theme functionality allows users to easily upgrade their themes without worrying about losing any custom styling that they have added to their site.
Often beginners confuse parent themes with theme frameworks. All WordPress theme frameworks are parent themes however not all parent themes are theme frameworks. In theory all WordPress themes can be a parent theme. However in order for a parent theme to be a framework, it needs to offer the ability for developers to modify and customize the core functionality of the theme without modifying any of the core theme files. This is usually done by using hooks and filters.
We recommend users to create child themes when you want to have custom styling however if you are trying to go beyond the basic CSS changes and want to add tons of additional functionality, then we recommend that you use a proper theme framework for building a child theme.