These grid sizes enable you to change the layout of columns to best match different screen widths and devices__ the essence of responsive design.
xs  :  for the smallest screen widths like smartphones < 768 px
sm  : for small screen widths like smartphones and tablets >= 768 px
md  :  for medium screen widths like tablets and laptops >= 992 px
lg  :  for large screen widths like desktops >= 1200 px.
