The most important verse in the Sri Guru Granth Sahib, the Sikh holy book, is the Mul Mantar: "God is One, His Name is True, the One who Does, Fearless, Hateless, Deathless, Birthless, Self-Enlightened, Guru's Gift; Pray!" The True (Sat = what is, what endures, what is sure) is a major part of Sikh religious belief, and, indeed, much of its doctrine can be construed as a meditation on Truth. The word occurs in Sikh greetings like Sat Naam (His Name is True) and Sat Siri Akal (Truth is Highest and Deathless). It is in great measure because it is so emphatic on Truth, as such, that Sikhism is monotheistic. The Sikh moral ideal is to be in accordance with Truth. The role of the Guru is to unite the student with Truth, as Guru Nanak says of the Guru (SGGS 17.14):
If it pleases Him, I bathe in the Pool of Truth, and become radiant and pure.
God is Truth and the Lover of Truth, and the fulfillment of human life is union with Truth.
This is often put in terms of light (jot). The Guru Granth presents each human being as existing by the gift of God, and at the fundamental heart of who they are is a light derived from divine Light. All of Sikh practice is concerned, directly or indirectly with reuniting the light within with the divine Light. The Sri Guru Granth Sahib is essentially a hymnbook; it is sung. It is also the perpetual Guru, carrying forward the light of Guru Nanak, and by this light manifesting the divine Light. So by singing its hymns, one unites oneself to the Guru, and thus uplifted one contemplates through the Guru the divine Light from which one comes; in doing this, one purifies oneself, and so makes oneself fit to unite again with the divine Light after death. This basic idea of our light uniting with divine light, joti jot samana, is found everywhere in Sikh thought, and is that which corresponds to moksha in Hinduism, liberation from the cycle of reincarnation.
An old poem of mine trying to capture various elements of Sikhism in a short form (Amritsar, of course, is the Sikh holy city):
Although the sea divides us, in Amritsar I stand;
my heart rests in the warmth of its nectar-golden sand.
In a vessel, clay and calm, made by the guru's hand,
I feel blessing pouring down: for in Amritsar I stand.
When time pools all around me like some silent sarovar,
I am in Ramdaspur; and, whether near or far,
my heart is by those waters as they shine beneath the stars
around the golden temple of blessed Amritsar.
When trouble overtakes me I flee to the fort of steel,
I shelter in the city with the sacred pools that heal,
I search for the jot of light where the psalms of gurus peal:
this world is all mirage, but Amritsar is real.