Web frameworks are designed specifically for the purpose of making your life as a developer or designer easier. They allow you to avoid typing in some of the code over when you are moving from a project to a project.
Normally, a framework would provide libraries for database access, managing sessions and cookies, display your HTML markup through templates, and encourage the developer to reuse the code. The framework is somewhat different from a content management system, like WordPress, for example. Framework is not so specific, and it is more like a collection of tools that you can use for your site-building.
Similar to this instance of reusing the code, web framework can do the same thing with your HTML markup. If you create or use a templating system provided by the framework, you can apply your instructions for footer or header, or other elements that you use repeatedly in your sites, to all the future sites.
For experienced developers, frameworks save a lot of time and allow many possibilities to experiment on details on their website without having to bother with long lines of code over and over again.
For inexperienced developers, the selection of frameworks can be tough, because all of them have many attractive features, and it is difficult to be able to spot the best one right away. Depending what you want from your website, you should try to focus on finding the one that best suits your needs as a designer. In case you are struggling with it, I suggest you to visit my posts, where I talk about best frameworks on the market and how to choose from them.