In Nichiren Daishonin's Buddhism, the term Myoho-renge-kyo means the entity of the Mystic Law itself, or Nam-myoho-renge-kyo. 

Dictionary of Buddhist Terms and Concepts

What Nichiren Daishonin wrote about Myoho-Renge Kyo:

"If you wish to free yourself from the sufferings of birth and death you have endured through eternity and attain supreme enlightenment in this lifetime, you must awaken to the mystic truth which has always been within your life. This truth is Myoho-renge-kyo. Chanting Myoho-renge-kyo will therefore enable you to grasp the mystic truth within you." 

"Myoho-renge-kyo is the king of sutras, flawless in both letter and principle. Its words are the reality of life, and the reality of life is the Mystic Law (Myoho). It is called the Mystic Law because it explains the mutually inclusive relationship of life and all phenomena. That is why this sutra is the wisdom of all Buddhas."

"However, even though you chant and believe in Myoho-renge-kyo, if you think the Law is outside yourself, you are embracing not the Mystic Law but some inferior teaching."

"Therefore, when you chant the Mystic Law and recite the Lotus Sutra, you must summon up deep conviction that Myoho-renge-kyo is your life itself."

from "On Attaining Buddhahood"

Myo "While I was thinking thus, a golden Buddha suddenly appeared in the hell of incessant suffering ... As my agony subsided a little, I joined my hands together in prayer and asked him what kind of Buddha he was. The Buddha replied, I am the character myo..." Major Writings, Vol. 4, p. 309 
Explanation of the Chinese characters for Myo.
  Ho "We will not be satisfied until justice rolls down like waters and righteousness like a mighty stream." Dr. Martin Luther King Jr. 
Explanation of the Chinese characters for Ho.
Ren "The lotus plant resembles the principle of Myoho-renge in that it simultaneously contains both cause and effect." Major Writings, Vol. 7, p. 66 
Explanation of the Chinese characters for Ren.
   Ge The Sutra says "Mandara flowers fall like rain," (U mandara ke, Gongyo book, p. 33), covering the believer in blessings. 
Explanation of the Chinese characters for Ge.
Kyo "Once you realize that your own life is the Mystic Law, you will realize that so are the lives of all others.  That realization is the mystic kyo..." Major Writings, Vol. 1, p. 5 
Explanation of the Chinese characters for Kyo.