“Dhammo maṅgalaṁ” is a famous line from the Jain Maṅgal Path / Ātmarakṣā Stotra. The full line in Prakrit is:

> Chattāri maṅgalāṁ, Arihantā maṅgalaṁ, Siddhā maṅgalaṁ, > Sāhū maṅgalaṁ, Kevali‑paṇṇatto Dhammo maṅgalaṁ.

Simple word‑by‑word meaning

• Chattāri maṅgalāṁ – These four are auspicious (mangalik).
• Arihantā maṅgalaṁ – Arihants are auspicious.
• Siddhā maṅgalaṁ – Siddhas are auspicious.
• Sāhū maṅgalaṁ – Sādhus (saints/monks) are auspicious.
• Kevali‑paṇṇatto Dhammo maṅgalaṁ –

The Dharma (true religion) propounded by the Kevalī (omniscient Lord) is auspicious.

Overall meaning (arth in simple English)

Among all things that bring real good and purity to the soul, four are truly auspicious:

1. Arihant – the living omniscient Tirthankaras who have destroyed inner enemies like anger, pride, deceit, greed.
2. Siddha – completely liberated souls, forever free from karma.
3. Sādhu / Sādhvī – monks and nuns who walk on the path of renunciation and self‑discipline.
4. Dharma taught by the Kevalī – the pure path of Right Faith, Right Knowledge, and Right Conduct shown by the Omniscient Ones.

“Dhammo maṅgalaṁ” means: The true Dharma revealed by the Omniscient is itself the highest maṅgal – the most beneficial, purifying and auspicious for the soul.

This understanding is common to both Śvetāmbara and Digambara traditions, though the exact recitation style may vary slightly.

You can read a related stotra with this line and its meaning here