Om is considered to be the primeval sound, the sound of the universe, the sound from which all other sounds are formed.
Om is therefore a sound symbolizing reality. It represents everything in the universe, past, present, and future. It even represents everything that is outside of those three times. It therefore represents both the mundane world of time in which the mind normally functions, and the world as perceived by the mind that is awakened and that experiences the world timelessly. It represents both enlightenment and non-enlightenment.
You could regard Om as being the equivalent of white light, in which all of the colors of the rainbow can be found.
Shanti (Pali: Santi) simply means “peace”. It’s a beautiful meaning and also a very beautiful sound. The shanti is repeated three times, as are many chants in Buddhism. In Buddhism as well as in Hinduism the threefold Shanti is generally interpreted as meaning the Threefold Peace in body, speech, and mind (i.e. peace in the entirety of one’s being).
Hindu teachings typically end with the words Om shanti shanti shanti as an invocation of peace, and the mantra is also used to conclude some Buddhist devotional ceremonies.