Suppose you have three columns, left-sidebar, main-content, right-sidebar. when the page is opened on small screen size the left-sidebar will be stacked at the top followed by the main-content and right-sidebar.
What if you wanted to get the main-content at the top followed by the left-sidebar and right-sidebar?
This can be accomplished by the bootstrap column push and pull classes.
<div class="row">
<div class="col-sm-8 col-sm-push-2">
<div style="background-color:red;color:#fff">main-content</div>
</div>
<div class="col-sm-2 col-sm-pull-8">
<div style="background-color:blue;color:#fff">left-sidebar</div>
</div>
<div class="col-sm-2">
<div style="background-color:blue;color:#fff">right-sidebar</div>
</div>
</div>