In his great treatise, Kyo Gyo Shin Sho (Teaching, Practice, Faith and Realization of the Pure Land Way), Master Shinran talks about the Nembutsu, "Namu Amida Butsu", as the GREAT PRACTICE.
In the largest sense, the Nembutsu represents Amida Buddha's Great Practice on our behalf through the power of His Primal Vow to save all suffering beings EVERYWHERE.
Now in True Shin Buddhism we only have one practice that we as bombu (spiritual idiots) "do", and that is Listening Deeply. We open our minds and hearts to the Dharma, we aspire to Buddhahood, and we interact with a true teacher who is a person of Shinjin.
The Primal Vow says,
If, when I attain Buddhahood, sentient beings in the lands of the ten quarters who sincerely and joyfully entrust themselves to me, desire to be born in my land, and call my Name, even ten times, should not be born there, may I not attain perfect Enlightenment. Excluded, however, are those who commit the five gravest offenses and abuse the right Dharma.
All that is required according to the Primal Vow is ENTRUSTING our entire karmic destiny to Amida Buddha to be born in Amida's Pure Land as a Buddha ourselves.
Namu Amida Butsu has two parts: "Namu" means "I take refuge", and "Amida Butsu" means "in Amida Buddha."
That is one great meaning of the Nembutsu. It is the BASIC meaning.
For the person who is seeking salvation, the Nembutsu means "Save me, Amida Buddha." It means that the seeker is opening her/himself to Amida's saving Light and Compassion. It is the seeker responding to Amida's Call and opening him/herself to Amida Buddha's Other Power.
Another meaning is, "Namu" represents me, the person who is saved, and "Amida Butsu" represents that which saves me, i.e. the Other Power of Amida Buddha. Once I am saved, my bombu "self" and Amida Buddha are as one. I am, as Master Shinran said, like Bodhisattva Maitreya, in the stage next to Buddhahood, for when I die I will become a Buddha just as Maitreya's next birth will be as a Buddha.
But there is another meaning of Namu Amida Butsu. When one receives Shinjin, one feels tremendous gratitude for being saved by Amida and granted the assurance of being reborn in His Pure Land as a fully enlightened Buddha.
Thus, for the person of Shinjin, Namu Amida Butsu means THANK YOU, AMIDA BUDDHA. It is the Nembutsu of Gratitude.
Finally, the Nembutsu is itself THE NAME OF THE BUDDHA. When we say that we are "Saying the Name" we are saying "Namu Amida Butsu," which means that Amida Buddha has fulfilled His Primal Vow on our part and has already assured our salvation. We only need to forsake all doubts and take refuge in Amida's Primal Vow of Other Power and be saved. This comes about through Listening Deeply with a true teacher.
Or as Master Shinran said, "Just say the Nembutsu and be saved by Amida. Nothing else is involved."
It all comes down to that. Listening Deeply, and Saying the Name. Listening Deeply and Saying the Name.
Or as my mentor Paul Roberts often says, "Before SHINJIN, Listen Deeply; After SHINJIN, Listen Deeply."
Namu Amida Butsu
Thank You, Amida Buddha!