How to get the Description of a Magento\Catalog\Model\Product?

In Store > Attributes > Product > Description > Storefront Properties set Visible on Catalog Pages on Storefront and Used in Product Listing to TRUE.

In your code list.phtml you can use:

<?php echo $this->helper('Magento\Catalog\Helper\Output')->productAttribute($_product, $_product->getDescription(), 'description') ?>

or

<?php echo $_product->getDescription() ?>