When enabling the "Hey Siri" function, iOS walks you through a brief voice training session. Simply say "Hey Siri" three times in a quiet location, answer a few questions, and the software learns your own unique voice.
After running through the steps and plugging in my iPad -- only the iPhone 6s and 6s Plus have the feature when not plugged in -- Siri responded to my commands and only my commands. My wife tried several times to activate Siri but no dice.
Other platforms have had this type of feature before, so it's not innovative by any means. In fact, it's overdue from that perspective. Regardless, it will make iOS devices a little more useful with voice recognition tailored to each individual user.