q2a automatically adds meta tags when on question page, but if you want to show category as meta descripton on other page then you can use this.
Open head.php and insert below code inside <head>
<?php $current_cat = ra_get_cat_desc(qa_request(2)); ?>
<?php if(is_array($current_cat)): ?>
<meta name="description" content="<?php echo substr($current_cat['content'], 0, 25); ?>">
<meta name="keywords" content="<?php echo $current_cat['title']; ?>">
<?php endif; ?>
