WordPress theme developers have created some schemes and processes to make theme development faster and easier. Some develop an HTML pages first and then develop the WordPress theme of it.
This way they have to re-write all codes from HTML to PHP. This time of developing WordPress theme can be reduced by a other way.
You can start with the WordPress codes and develop your design around that. This increases the speed of the coding process because you just type in a language that WordPress understands.
A Theme Framework is a code library that can be used to speed up the process of WordPress Theme Development. It can be Another Theme that act as a Starter or a Parent Theme.
Theme Frameworks are those tools that make the process of creating WordPress function. A Developer need not to re-write the code again, instead he can use these code library or Tools to develop a another Theme.