Of course you can edit the boardindextemplate to add descriptions for categories..
find
<foreach loop="categories:$cat_data as $_data">
<if test="cat_has_forums:|:is_array( $_data['forum_data'] ) AND count( $_data['forum_data'] )">
<div id='category_{$_data['cat_data']['id']}' class='category_block block_wrap'>
<h3 class='maintitle'>
<a class='toggle right' href='#' title="{parse expression="sprintf( $this->lang->words['toggle_cat'], IPSText::striptags( IPSText::htmlspecialchars($_data['cat_data']['name'] ) ) )"}">{parse expression="sprintf( $this->lang->words['toggle_cat'], $_data['cat_data']['name'] )"}</a> <a href="{parse url="showforum={$_data['cat_data']['id']}" seotitle="{$_data['cat_data']['name_seo']}" template="showforum" base="public"}" title='{parse expression="sprintf( $this->lang->words['view_cat'], IPSText::striptags( IPSText::htmlspecialchars($_data['cat_data']['name'] ) ) )"}'>{$_data['cat_data']['name']}</a>
</h3>
change to
<foreach loop="categories:$cat_data as $_data">
<if test="cat_has_forums:|:is_array( $_data['forum_data'] ) AND count( $_data['forum_data'] )">
<div id='category_{$_data['cat_data']['id']}' class='category_block block_wrap'>
<h3 class='maintitle'>
<a class='toggle right' href='#' title="{parse expression="sprintf( $this->lang->words['toggle_cat'], IPSText::striptags( IPSText::htmlspecialchars($_data['cat_data']['name'] ) ) )"}">{parse expression="sprintf( $this->lang->words['toggle_cat'], $_data['cat_data']['name'] )"}</a> <a href="{parse url="showforum={$_data['cat_data']['id']}" seotitle="{$_data['cat_data']['name_seo']}" template="showforum" base="public"}" title='{parse expression="sprintf( $this->lang->words['view_cat'], IPSText::striptags( IPSText::htmlspecialchars($_data['cat_data']['name'] ) ) )"}'>{$_data['cat_data']['name']}</a>
<br>My Cat description here.
</h3>
The easiest way would be to add the description to the language files something like $this->lang->words['cat10_description'] and create one for each cat replacing the 10 with the cat id number. Then adding that instead of a bunch of if statements.