Here you can see the sample of screen size.
Size | Class Prefix
==========================
Extra small | .col-
Small | .col-sm-
Medium | .col-md-
Large | .col-lg-
Extra large | .col-xl-xs = extra small screens (Mobile)
<div class="col-xs-6">Column 1</div>
<div class="col-xs-6">Column 2</div>sm = small screens (Tablets)
<div class="col-sm-6">Column 1</div>
<div class="col-sm-6">Column 2</div>md = medium screens (Laptos)
<div class="col-md-6">Column 1</div>
<div class="col-md-6">Column 2</div>lg = large screens (Desktops)
<div class="col-lg-6">Column 1</div>
<div class="col-lg-6">Column 2</div>