Theme frameworks are prewritten bits of code that are used by developers in order to create new themes. If you’re not planning on coding a new theme, then you won’t need to understand what theme frameworks are.
Long ago, WordPress themes were custom written from scratch, making it very hard for developers to upgrade their theme designs without having to begin all over at the beginning. Today, theme frameworks serve as a way to easily copy over the elements of an existing theme in order to build a new theme.
Theme frameworks are a bundle of different files that are contained within a single folder. These different files are sometimes known as a “code library,” and contain all of the essential parts for building a theme. In essence, theme frameworks allow developers to “copy-paste” parts of an existing theme in order to build a new theme. This way, they don’t have to rewrite every element from scratch.
WordPress invented theme frameworks so that designers can easily set up child themes based on an existing parent theme template. Designers can then make style modifications inside the child theme while preserving the core functionality in the parent theme.
Theme frameworks were primarily invented in order to make it easier to apply security upgrades for themes. By keeping the core functionality in one file folder (the theme framework), applying upgrades or fixing bugs can be done once, and the child theme(s) will then automatically be updated as well.