A framework is literally a frame upon which a web designer adds the design elements. A WordPress Theme frameworks is a skeletal frame set with HTML and CSS with all the WordPress code embedded within it, upon which a designer can paint the website walls.
Most WordPress designers and developers either create their own WordPress Theme frameworks or use one already created for every WordPress site they develop. It’s important for them to be familiar with all the HTML and CSS design elements so they can quickly apply the design elements, whether from scratch, wireframe, or from a Photoshop to WordPress conversion. The more you understand how a Theme framework functions, the faster the design work can be applied.