Please help me understand the below code on the main page.

Summary:

Below is the code on the main page

----

<oj-drawer-layout id="navDrawer" role="navigation"

    start-display="[[ $application.responsive.smOnly ? 'overlay' : 'reflow' ]]">

    <oj-vb-fragment slot="start" id="shell-drawer-child" bridge="[[vbBridge]]" name="shell-drawer-child">

     <oj-vb-fragment-param name="currentFlow" value="{{ $page.currentFlow }}"></oj-vb-fragment-param>

    </oj-vb-fragment>

    <div id="pageContent" class="oj-flex oj-sm-flex-direction-column oj-sm-flex-wrap-nowrap fixed-header">

     <!-- Header: Includes branding and menu items -->

     <oj-vb-fragment id="shell-header" bridge="[[vbBridge]]" name="shell-header-child">

     </oj-vb-fragment>

     <div class="oj-flex oj-sm-flex-direction-column oj-sm-flex-1 oj-sm-flex-wrap-nowrap">

      <!-- Renders flow content -->

      <oj-vb-content class="vb-pages-module oj-flex-item oj-flex" config="[[vbRouterFlow]]">

      </oj-vb-content>

     </div>

    </div>

   </oj-drawer-layout>

---

Below is the part of the code from above

------

    <oj-vb-fragment slot="start" id="shell-drawer-child" bridge="[[vbBridge]]" name="shell-drawer-child">