A WordPress parent theme is a theme from which a child theme inherits its style and functionality:
The implementation of this feature on the WordPress platform is that it provides designers and theme developers with a way to alter much larger and more elegant themes, enabling the creation of child themes.
A WordPress Parent theme will subsequently pass down all of its attributes in terms of features, functionality, and style onto a WordPress Child theme. From there the sub or child theme can undergo modifications to alter certain functions of a Parent theme without really changing or modifying a Parent theme.
The parent/child theme relationship provides developers, designers, and users a way to efficiently upgrade themes without the hassle of losing custom designs and styles already implemented onto the site.
Confusion with parent themes often come by mistaking parent themes for theme frameworks. WordPress theme frameworks are very similar and yes, all theme frameworks are parent themes, but not all parent themes are frameworks.
In order for a Parent theme to be considered a theme framework, it must offer a way for designers and developers to make changes and alterations to the core functions of the theme without actually changing the core theme files. Typically, this is accomplished by using hooks and filters.
It is suggested that users start by creating child themes when looking to add custom styling and design, however, if you intend on making a lot of changes past the CSS basics, then you should develop the proper WordPress theme framework for creating a child theme.