Responsive grids and components mean that your site works on all devices.

More and more people are accessing websites and applications via mobile devices. It 

Responsive web design refers to sites which work on a variety of devices, including mobile, tablet, desktop and any number of other devices. A site needs to be able to display content and enable user interactions in a wide range of screen sizes and device capabilities. In practical terms this means displaying content and layouts in such a way that design is able to adapt to the device it is in.

Bootstrap is an open source project which provides a standardised framework for building responsive theme. It provides a grid system which can be customised for screen width, as well a wide range of other components which brings consistency and predictability to the user experience.