The biggest advantage of Bootstrap is its Grid System. It uses a 12 column layout. You specify how many columns an element should take on different screen sizes using col-xs, col-md, col-lg.

A 3-Column Layout

<div class="container">
  <div class="row">
    <div class="col-12 col-md-4">
      <!-- Takes full width on mobile (12) -->
      <!-- Takes 4/12 (one third) on desktop -->
      <h3>Column 1</h3>
    </div>
    <div class="col-12 col-md-4">
      <h3>Column 2</h3>
    </div>
    <div class="col-12 col-md-4">
      <h3>Column 3</h3>
    </div>
  </div>
</div>

Check out the Live Demo! Resize your browser window and watch how the columns smartly stack on top of each other dynamically.