Within templates $this->helper()$this->helper()
gets translated to Mage::helper()Mage::helper()
So this code means this is a call to Mage::helper('customer')Mage::helper('customer')
, which gets translated to Mage_Customer_Helper_DataMage_Customer_Helper_Data
(or any class that rewrites it) in the backend.
If you're curious how this is done have a look at Mage_Core_Model_Config::getHelperClassName()Mage_Core_Model_Config::getHelperClassName()