If you want to get bundle child product description on bundle parent product detail page in ‘DETAILS’ tab.

You can override attribute.phtml file in your theme and put below code at bottom of the file

<?php 
if($_product->getTypeId() === 'bundle'){ 
 $objectManager = MagentoFrameworkAppObjectManager::getInstance();
 $product = $objectManager->get('MagentoCatalogModelProduct')->load($_product->getId()); 
 $selectionCollection = $product->getTypeInstance(true)->getSelectionsCollection($product->getTypeInstance(true)->getOptionsIds($product),$product);
 foreach ($selectionCollection as $proselection) { 
 $option_product = $objectManager->get('MagentoCatalogModelProduct')->load($proselection->getId()); ?>
 <div class="bundle-item-description"> 
 <h3><?php echo $option_product->getName() ?></h3>
 <p><?php echo $option_product->getDescription(); ?></p>
 </div>
 <?php }
} ?>