Om Tat Sat is a mantra

Mantra

A mantra is a sound, syllable, word, or group of words that is considered capable of "creating transformation"...

 in Sanskrit

Sanskrit

Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 IT MEANS THESE ARE THE THREE WORDS OF THREE GOD’,

• Om
  Aum
  Om or Aum Om or Aum Om or Aum (also , written in Devanāgari as and as , in Sanskrit known as (lit. "to sound out loudly"), ', or ' (also as ') (lit. "Auṃ form/syllable"), is a sacred/mystical syllable in the Dharmic or Indian religions, i.e...
  
  refers to the Supreme Infinite Spirit
  Brahman
  In Hinduism, Brahman is the one supreme, universal Spirit that is the origin and support of the phenomenal universe. Brahman is sometimes referred to as the Absolute or Godhead which is the Divine Ground of all being...
  
   or Person
  Bhagavan
  Bhagavan, also written Bhagwan or Bhagawan, from the Sanskrit nt-stem literally means "possessing fortune, blessed, prosperous" , and hence "illustrious, divine, venerable, holy", etc.In some traditions of Hinduism it is used to...
  
  . Om
  Aum
  Om or Aum Om or Aum Om or Aum (also , written in Devanāgari as and as , in Sanskrit known as (lit. "to sound out loudly"), ', or ' (also as ') (lit. "Auṃ form/syllable"), is a sacred/mystical syllable in the Dharmic or Indian religions, i.e...
  
   represents the Shabda Brahman
  Brahman
  In Hinduism, Brahman is the one supreme, universal Spirit that is the origin and support of the phenomenal universe. Brahman is sometimes referred to as the Absolute or Godhead which is the Divine Ground of all being...
  
  .
• Tat MEANS THE SECOND GUPTT. NAME OF GOD."AAKSHAR BHAHAM" OM+TAT= SAT-NAAM.
• Sat MEANS'THE THIRD GUPTT NAME OF GOD. "PURAN AAKSHAR BHAHAM",OM+TAT+SAT=SAARNAAM.

Om Tat Sat also reflects THAT THESE ARE THE NAME OF THREE GOD AND EVERY HUMAN BEING SHOULD LEARN THIS.

These sacred syllables were integral to religious and material goodness and spiritual pursuit in Vedic

Vedic

Vedic may refer to:* the Vedas, the oldest preserved Indic texts** Vedic Sanskrit, the language of these texts** Vedic period, during which these texts were produced** Vedic pantheon of gods mentioned in Vedas/vedic period...

age.

Variations in Usage

Hari Om Tat Sat (Hari means vishnu,om means bhram , tat means gupt word of par bhram, sat means the gupt name of puran bhram ) the name of two god is known by true saint, balance are fake saint.
the nanak ji clarifies in holy guru granth sahib that the sat naam is very important name and every body should learn this till death.