In short: the Three Jewels (or Triple Gem) are the Buddha, the Dhamma and the Saṅgha: the teacher, the teaching and the community. Buddhists "go for refuge" in them, and the Buddha taught that recollecting them removes fear (SN 11.3).
The refuge chant
Buddhaṁ saraṇaṁ gacchāmi. Dhammaṁ saraṇaṁ gacchāmi. Saṅghaṁ saraṇaṁ gacchāmi.
I go for refuge to the Buddha. I go for refuge to the teaching. I go for refuge to the Saṅgha.
Full chant and meaning · Why three bows
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How to become a Buddhist · Morning practice
Questions people ask
What are the Three Jewels?
The Buddha (the awakened teacher), the Dhamma (his teaching) and the Saṅgha (the community of his disciples). Buddhists 'go for refuge' to all three.