Brihadaranyaka Upanishad is the primary (Mukhya) Upanishad amongst all the Upanishads. It is the commentary on White (Shukla) Yajur Veda and is associated with Shatapatha Brahmana. The last five chapters from the fourteen volume of Shatapatha Brahmana is the Brihadaranyaka Upanishad. It is supposedly written by Sri Yogeeswara Yagnavalkya.
Brihadaranyaka Upanishad is as old as the Shukla Yajur Veda, exact time is unknown. Brhat means big and aranya means forest; together put, it means a book that has many chapters and which should be read in a forest, where there is solitude and peace. It contains deep spiritual knowledge and so it better to read it when there is inner and external peace.
In the ancient times, sages would prefer to meditate in forests; it may be with the same motive. Another reason, why it is called as Brihadaranyaka may possibly be because sage Yagnavalkya wrote it in the forests. That era, sages spent lot of their time in the forests.
There are stories, which have dual purpose like one about the senses and the soul. It reveals the nature of the senses and the soul within every individual. And at the same time teaches how to be selfless and detached like the soul, to achieve victory against negative forces. Similarly, there are spiritually enlightening stories that if grasped in its correct sense can dissolve the mysteries of the universe.
All the Upanishads are to a certain extent similar to each other. The basic wisdom put forth is about how the imperishable soul is actually a fragment of the divine and every human should work towards realizing this truth instead of running after materialistic pleasures.