Bootstrap 3's grid system introduced the concept of having a different grid layout dependent on the browser's size.
Adjusting the browsers size will alter the grid layout and will highlight the current CSS being applied.
This will provide a visual representation of the link between the browser size and the bootstrap grid css class being applied.
Bootstrap 3's grid system also allows us to hide and show columns depending on the width of a page. This can be achieved using the visible-xx or hidden-xx CSS classes.