Very simple, the terminology is most likely confusing you.
Here is a breakdown of a common, simple example of Bootstrap using containers.
Code
<div class="container">
<div class="col-md-6">
Half of the 'container'
</div>
<div class="col-md-3">
Quarter of the container
</div>
<div class="col-md-3">
2nd quarter of the container
</div>
</div>
Using simple containers like this allows you to easily manipulate certain things. For example if I wanted to add padding to the overall container, I can easily target that with the CSS
Code
.container{
padding-top: 20px;
}
Therefore this would bring the whole container (with the elements inside) down 20 pixels.
Hope this helps.