“Tu sumar sumar re Shantinath sukhdai re” is a popular Gujarati stavan/bhajan line in praise of Bhagwan Shantinath (16th Tirthankar).

Simple meaning (arth): “O Shantinath, giver of true happiness, I remember you again and again.”

Notes for devotion:

• This is a bhakti stavan (not a canonical sutra). It expresses repeated smaran (remembrance) of Shantinath Bhagwan, whose very nature is shanti (peace) and kevalanand (supreme bliss).
• Both Shwetambar and Digambar Jains sing such stavans; there is no doctrinal difference in the meaning of this line.

If you wish to connect this bhajan to a traditional Jain stotra of Shantinath with authentic verses and meanings, see Laghu Shanti Stavan/Stuti (dedicated to Bhagwan Shantinath) here: