rows and columns and bootstrap code example

Example 1: bootstrap col-md-5 center

<div class="container">
  <div class="row">
     <div class="col-sm-5 mx-auto">
        Centered
    </div>
  </div>
</div>

Example 2: bootstrap screen sizes

The Bootstrap grid system has four classes:
xs (for phones - screens less than 768px wide)
sm (for tablets - screens equal to or greater than 768px wide)
md (for small laptops - screens equal to or greater than 992px wide)
lg (for laptops and desktops - screens equal to or greater than 1200px wide)