Om Shanti Shanti Shantihi
Let there be peace, peace, peace!
The word Shanti means peace in Sanskrit. It features three times in this mantra. This repetition is to removing obstacles from three realms.
Adhi-Bhautika (the physical realm): This realm can create obstacles that could physically harm you.
For instance, wild animals, natural disasters, people, etc.
Adhi-Daivika (the Divine realm): This realm can create obstacles from the intangible.
For instance, evil spirits, karma, demigods, etc.
Adhyaatmika (the realm within us): This realm can create obstacles from within. For instance, stress, pain, illness, disorders, lethargy, anxiety, etc.
Some consider that the three 'Shanti's are to request peace from the three core Gods of the Universe: Brahma, Vishnu, and Shiva.
Some even believe that the Shanti mantra invokes the three core Goddesses: Lakshmi, Saraswati, and Parvati.
Invoking peace from Lakshmi ensures that there will be no stress financially. Being blessed by Saraswati ensures that your knowledge and creativity won't face any obstacles.
And Parvati's blessings will give you the nurturing power to sustain your well-being.
That is why Om Shanti is often assimilated with a lot of Hindu prayers and chants. It is often used at the beginning and/or the end of the verse of other ceremonious mantras.
Similarly, in many Buddhist ceremonies, the mantra is fused within other mantras.
In Buddhist practices, the three repetitions of Shanti signifies peace in mind, speech, and body.