So what is respond.js ?
Most of the current web sites are designed and developed to be responsive. To be able to adopt various screen sizes on various devices, specially mobile devices. This is archived mostly using CSS3 media queries. But older browsers does not support CSS3 will fail to display your web site properly. So to overcome this a fast and lightweight framework was developed and its called respond.js
You can find the GitHub repository at https://github.com/scottjehl/Respond