Here you can control the responsive behaviour of your site (which elements appear on screens of different sizes).  By default most non essential items are hidden on mobile devices, being set to appear only on larger screens.  You can make elements appears on smaller screens by choosing to display them on "Extra Small" or "Small screens".  To gain a better understanding of how these settings work please refer to the Bootstrap '<a href="{$a}" target="_blank">Display property</a>' documentation.