Lord Balarama appeared in the womb of Devaki as her seventh son. She could understand the Lord Vishnu had entered her womb. While she was jubilant about it, she was also anxious that Kamsa would kill Him like he had her six previous children.
Krishna ordered His yogamaya potency to transfer Balarama who is actually shesha the multi-hooded serpent on whom Vishnu rests, from the womb of Devaki to Rohini’s womb. Rohini was also one of Vasudeva’s queens but was living in the home of Nanda Maharaja in Gokula out of fear of Kamsa’s atrocities.
Lord Krishna stated that due to being forcibly attracted to the womb of Rohini, He would be known as Sankarshana. He would be the source of all spiritual strength, bala that enables one to attain the highest bliss called ramana. Therefore the plenary portion Ananta (or Shesha) would be known as Sankarshana or Balarama.
It was thus that Balarama was born as the son of Rohini.
Bala or spiritual strength is essential for attaining self-realization and Lord Balarama is the giver of spiritual strength. He is the gravitational force that sustains the planets in their universal positions. The one who bestows spiritual power He is the original spiritual master (Guru).