It initiates the resource model this model uses to commit data to the database.

So in this case that would be the file app/code/[local|community]/Magentotutorials/Weblog/Model/Resource/Blogpost.php

It initiates the resource model this model uses to commit data to the database.

So in this case that would be the file app/code/[local|community]/Magentotutorials/Weblog/Model/Resource/Blogpost.php

[EDIT] to answer @easymoden00b question, the _init method can be found in Mage_Core_Model_Abstract which directly calls _setResourceModel in the same class

/**
 * Set resource names
 *
 * If collection name is ommited, resource name will be used with _collection appended
 *
 * @param string $resourceName
 * @param string|null $resourceCollectionName
 */
protected function _setResourceModel($resourceName, $resourceCollectionName=null)
{
    $this->_resourceName = $resourceName;
    if (is_null($resourceCollectionName)) {
        $resourceCollectionName = $resourceName.'_collection';
    }
    $this->_resourceCollectionName = $resourceCollectionName;
}

This stores the Resource item and collection class names for further use when retrieving and writting to and from the database