It's not a mantra.
It's a Buddha's name in Tibetan.
Chom-Den-De / De-Zhin Sheg-Pa /Dra Chom-Pa /
Bagavan / thatagatha / Arhate
Yang Dag-Par / Dzog-Päi Sang-Gye /
Samya Sam Buddha
Ngo-Wa Dang Mön-Lam Tham-Che / Rab-Tu Dru-Pä / Gyäl-Po-La /
(something not sure) and all aspirations/ boardly(?) accomplished/ king
I think it might be from a sutra, not very sure.
there are other multiply merits mantras; a famous one is the utmost essence mantra from 'Dhāranī of the Adorned Place of Bodhi ' or Bodhimandalalaksālamkāra dharani.
By reciting this mantra after your prayers, all your wishes will be actualized. Recite this mantra to multiply the benefits by 100,000 times.
Chom-Den-De / De-Zhin Sheg-Pa /Dra Chom-Pa /Yang Dag-Par / Dzog-Päi Sang-Gye / Ngo-Wa Dang Mön-Lam Tham-Che / Rab-Tu Dru-Pä / Gyäl-Po-La / Chag-Tshäl Lo (3x)
Due to the power of the blessing of the eminent Buddhas and Bodhisattvas, the power of infallible dependent arising and the power of my pure special attitude, may all my pure prayers succeed immediately.
From Lama Zopa of FPMT