What is meant by praying two rak'ahs without an obligatory prayer?

The Messenger of Allah, peace and blessings be upon him, used to teach his companions the الاستخارة (istikharah) prayer for all matters, just as he would teach them a chapter from the Qur'an. The الاستخارة prayer consists of two rak'ahs, not from the obligatory prayers, after which one recites the الاستخارة supplication.