Namutthunam (Namutthunam / Namotthunam) is a Prakrit stotra that offers deep salutation to the Arihant–Tirthankars and Siddhas. Below is its simple overall meaning, line by line (arth in easy English):

1. “Namutthunam arihantāṇam bhagavantāṇam”

Bowing again and again to the Arihant Bhagwans – the enlightened, living Jinas.

1. “Ai-garāṇam, tittha-yarāṇam, sayam-sambuddhāṇam”

Salutation to those who started the Jain scripture tradition, who are Tirthankars (makers of the ford of dharma), and who attained enlightenment by their own effort.

1. “Purisuttamāṇam… purisa-vara-gandha-hatthīṇam”

Salutation to the best among humans, the lions among men, the most excellent lotus among men, and those strong and majestic like the best elephants – meaning the Jinas are supreme, fearless, and noble.

1. “Loguttamāṇam… loga-pajjoa-garāṇam”

Salutation to those who are higher than the whole world, leaders and benefactors of the world, who remove darkness and ignorance and light up the world with right knowledge.

1. “Abhaya-dayāṇam… bohi-dayāṇam”

Salutation to those who remove fear, give spiritual vision, show the right path, give refuge, and grant true awakening (right faith and enlightenment).

1. “Dhamma-dayāṇam… dhamma-vara-chāuranta-chakkavattīṇam”

Salutation to those who give dharma, preach dharma, protect and lead dharma, and who are universal monarchs in the realm of dharma in all directions.

1. “Appaḍihaya-vara-nāṇa-daṁsaṇa-dharāṇam, viyaṭṭa-chhaumāṇam”

Salutation to those who possess unobstructed, supreme knowledge and faith, whose radiance and aura are unique and incomparable.

1. “Jiṇāṇaṁ… muttāṇaṁ moagāṇaṁ”

Salutation to the victorious Jinas, who have crossed the ocean of saṁsār and help others to cross, who are enlightened and also givers of enlightenment, liberated and givers of liberation.

1. “Savvannūṇaṁ, savva-darisīṇaṁ… namo jiṇāṇaṁ, jia-bhayāṇaṁ”

Salutation to the all-knowing and all-seeing Jinas, who have attained a pure, peaceful, formless, infinite, eternal, unobstructed state (Siddhahood), free from rebirth – and who grant fearlessness to all living beings.

1. “Je a aīyā siddhā, je a bhavissaṁti… savve ti-viheṇa vandāmi”

I bow to all Siddhas – - those who have already become Siddha, - those who will become Siddha in the future, and - those who are Siddha now – paying respect to all of them in all three times.

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Essence in one line: Namutthunam is a praise and surrender to Arihant–Tirthankars and Siddhas, remembering their supreme qualities and praying that we may also walk their path and attain fearlessness and liberation.

Tradition note:

• Shwetambar: Widely used in Chaityavandan, also called Shakra-stava (because Indra recites it when a Tirthankar takes conception).
• Digambar: The exact stotra text may differ or is less commonly recited in daily lay practice, but the bhāv (devotion to Arihant–Siddha) is fully in line with Digambar doctrine.

You can read the full Namutthunam Sutra text with meaning here