Check out example codes for "bootstrap grid". It will help you in understanding the concepts better.

Code Example 1

col-12 col-sm-4 col-md-3 col-lg-12	col-xl-12

Code Example 2

<!----------------------- BOOTSTRAP GRID SYSTEM ------------------------>

This code will create 4 boxes placed side by side, which will be 
dynamically positioned, according to the size of the screen. We define 
the following behavior: 


<!-- Desktop Display: we have 4 columns per row (each occupies 3 units out of 12)  -->

class = "col-lg-3"   <!-- display large with 12/3 = 4 columns -->


<!-- Tablet Display: we have 3 columns per row (each occupies 4 units out of 12)  -->

class = "col-md-4"   <!-- display medium with 12/4 = 3 columns -->


<!-- Mobile Display: we have 2 columns per row (each occupies the 6 units of 12)  -->

class = "col-sm-6"  <!-- display small with 12/6 = 2 columns -->


<!-- CODE -->

  <div class="row">
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:red; border:1px solid black;">
      One of four columns
    </div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:yellow; border:1px solid black;">
      One of four columns
    </div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="background-color:green; border:1px solid black;">
      One of four columns
  	</div>
    <div class="col-lg-3 col-md-4 col-sm-6" style="border:1px solid black;">
      One of four columns
  	</div>
  </div>

Code Example 3

<div class="row">
  <div class="col-md-4">.col-md-4</div>
  <div class="col-md-4 offset-md-4">.col-md-4 .offset-md-4</div>
</div>
<div class="row">
  <div class="col-md-3 offset-md-3">.col-md-3 .offset-md-3</div>
  <div class="col-md-3 offset-md-3">.col-md-3 .offset-md-3</div>
</div>
<div class="row">
  <div class="col-md-6 offset-md-3">.col-md-6 .offset-md-3</div>
</div>

Code Example 4

<div class="container">
  <div class="row">
    <div class="col-sm">
      One of three columns
    </div>
    <div class="col-sm">
      One of three columns
    </div>
    <div class="col-sm">
      One of three columns
    </div>
  </div>
</div>

Code Example 5

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)

Code Example 6

.col-sm-3half, .col-sm-8half {
    position: relative;
    min-height: 1px;
    padding-right: 15px;
    padding-left: 15px;
}

@media (min-width: 768px) {
    .col-sm-3half, .col-sm-8half {
        float: left;
    }
    .col-sm-3half {
        width: 29.16666667%;
    }
    .col-sm-8half {
        width: 70.83333333%;
    }
}

Learn ReactJs, React Native from akashmittal.com