It’s a Gujarati stavan line. Sense of the line:

“Belakarm mātra terne, melvya mukti nā mahel.” Meaning: When only a tiny remnant of karma was left, they (those souls) attained the palace of liberation (moksha).

Simple explanation in Jain dharma:

• A soul is bound by eight types of karmas.
• Through right faith, right knowledge, and right conduct, along with tapas (austerity) and true restraint, karmas are shed (nirjarā).
• When karmas are reduced to the slightest residue and the ghāti-karmas are destroyed, keval-jñān arises; with the final shedding of the remaining aghāti-karmas, the soul attains moksha.
• The line poetically says: after reducing karmas to a mere trace, the perfected beings attained the abode of liberation.

Both Śvetāmbara and Digambara traditions agree on this core meaning about shedding karmas and attaining moksha; scriptural enumerations and narrative settings may differ, but the essence remains the same.

If you’d like to read a very simple overview of karma and moksha in Jainism, you can refer here: and here: