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addAttributeToFilter is a method that can be called on EAV collections in Magento. This includes product collections, category collections, customer collections and many more. In short, it adds a condition to the WHERE part of the MySQL query used to extract a collection from the database, therefore allowing you to filter the collection by custom conditions:

$_products = Mage::getResourceModel('catalog/product_collection')
   ->addAttributeToSelect(array('name', 'product_url', 'small_image'))
   ->addAttributeToFilter('sku', array('like' => 'UX%'))
    ->load();

To use with Fishpig (non EAV collections), you instead need to use addFieldToFilter, like so:

$posts = \Magento\Framework\App\ObjectManager::getInstance()->create('FishPig\WordPress\Model\ResourceModel\Post\Collection')
    ->addPostTypeFilter('post')
    ->setOrderByPostDate()
    ->addCategoryIdFilter(array($currentCategoryId))
    ->addIsViewableFilter()
    ->addFieldToFilter('id', array('neq' => $currentPostId))
    ->load(); ?>