[Lynne]

That looks like the forum.php page to me, yet you said "But No Changes in Forum page : want to display description as default", so I'm confused about where you want to remove it. But again, you need to put the template name in the source code using the method I posted and then find the template using that method. According you using that method, the template should be
<!-- BEGIN TEMPLATE: forumhome_forumbit_level2_post -->

So, just remove the description text from there:

<vb:if condition="$show['forumdescription']"><p class="forumdescription">{vb:raw forum.description}</p></vb:if>

(You could also just write a plugin to set $show['forumdescription'] for that template when on that page, but it may be easier for you to just edit the template if you don't know how to write plugins.)