Garbha Kalyanak in Jainism means the auspicious event of conception when a Tirthankar’s soul enters the mother’s womb.

It is one of the five Kalyanaks of a Tirthankar:

1. Garbha Kalyanak – conception
2. Janma Kalyanak – birth
3. Diksha Kalyanak – renunciation
4. Keval Gyan Kalyanak – omniscience
5. Moksha Kalyanak – liberation

Meaning

At the time of Garbha Kalyanak, very auspicious signs are believed to occur. In Jain tradition, this event marks the beginning of the final earthly life of a Tirthankar.

Spiritual importance

• It is a highly sacred moment.
• It indicates that a great soul has entered the human world for the welfare of all living beings.
• Devotees remember it with reverence and bhakti.

In simple words

Garbha Kalyanak = the holy conception of a Tirthankar.

If you want, I can also explain the five Kalyanaks of a Tirthankar in simple Jain language.