Read this post about an introduction to the Rasas.
At the beginning of the Kurukshetra war, faced with the dilemma of the inevitability of raising arms and spilling the blood of his relative, cousins and kinsmen, Arjuna is filled with despair and is completely despondent at the helplessness of his situation. So much so, that he even neglects his duty as a Kshatriya warrior and drops his weapons in his chariot and steadfastly refuses to pick them up and prepare for the war.
At this juncture, Krishna counsels Arjuna about his duties as a warrior and a Kshatriya prince, elaborating on a variety of philosophical concepts. To convince Arjuna of his words, Krishna also reveals his true divine form, his Viswaroopa to him. Arjuna experiences the vision of the Viswaroopa with divine vision endowed to him by Krishna.
Some descriptions of the Viswaroopa are thus – He has innumerable forms, eyes, faces, mouths and arms. All creatures of the universe are part of Him. He is the infinite universe, without a beginning or an end. He contains peaceful as well as wrathful forms.
Another description of the Viswaroopa goes thus – In this form, Krishna no longer looks familiar. He is neither cowherd nor charioteer. He is a magnificent being with countless legs and countless arms and countless heads, breathing fire, containing within himself all the things that exist – all the worlds, all the animate beings and inanimate objects. It is an awe inspiring sight. Containing everything that Arjuna imagined and also everything that was beyond Arjuna’s imagination.
Despite multiple descriptions of the Viswaroopa, one thing is for certain, Arjuna was so dazzled by the brilliance and the magnificence of this particular form of Krishna that all his doubts, fears and hesitation was immediately put to rest and he picked up his weapons and prepared for combat. The Viswaroopa convinced him that performance of one’s duty was of paramount importance. Arjuna was so overcome with faith, devotion and confidence in Krishna that all his doubts were assuaged immediately.
Thus, the episode with the Viswaroopa is a clear allusion to the bhakti rasa in the Mahabharata.