The term WordPress theme framework often refers to a code library that is used to facilitate development of a theme. In the old days of WordPress, there were some crucial problems with the way how themes were developed and maintained. There was no good way of upgrading WordPress themes without losing all the custom styling options. There was no way to prevent copying and pasting of the same functionality code in all themes. While these two issues might not seem like a problem to an average user, these can be disastrous for a few reasons.
What if you realized that the same code that you had been using in all of your themes had a security exploit. Even more important, what if this theme was something that you publicly released for others to download and customize. Yes, that’s chaos waiting to happen. The core team and the community decided to fix the issues mentioned above by introducing the concept of Parent Theme and Child Theme.
WordPress theme frameworks are intended to be used as a parent theme template where all the functionality resides. Developers can then create a child theme to add custom styling while leaving the functionality aspects of it to the framework. This allows for a centralized location where all the functions are hosted. If the core development team decide to deprecate a WordPress function, or there is a bug found in a specific theme framework, then it is extremely easy to push out an update without modifying anything the child theme has. This method allows you to keep the “framework” of your site strong without modifying how it looks.