what is widget "mobile responsive" used for?

The Mobile Responsive Widget description says this.

The widget automatically detects the device where the form is opened, and applies a Custom Style sheet that will make the fields of the form to resize their widths according to the device screen size.

There is no additional configuration required by the widget - Just add it to your form and the widget will do the rest.

link: http://widgets.jotform.com/widget/mobile_responsive

Hope this help. Let us know if you need further assistance.