This is a Sanskrit word Tamilized/Malayalamized. This is the term which means one who gives release to the manes.

As per the Dharma Shastras/Smritis, The dead people in a family are to be sent with properly coducted post death rituals for which a son born to the person is the most eligible. This involves performance of the 13-day ceremonies, by the son(eldest, if more tha one are there). This gives the pitR^is the "right" to travel to the pitR^iloka where they await "judgment" on whether to be despatched to naraka(Hell) or swarga(heaven). The rituals convert the status of the "preta"(the ethereal body) to "pitR^i"(the astral body) so that they could cross the vaitaraNI( a foul smelling river into which "helpless" atmas(actually their [karma-shariras] may fall into. The son's faithfully performing the rituals is supposed to help the karma-sharira of the deceased to move over and cross the vaitaraNI.

Further on after the Atma(karmasharira) reaches the pitR6i waiting room where it stays for an earth year, moves into the respective loka as the case may be. All the shraddham rituals and the tarpanams done for the manes(pitR^is) will aid them in moving further up the ladder of "evolution" to return back to earth after finishing their trysts with the adho-lokas and the swar-lokas. The length of stay in each of the lokas depends on the pApa-puNya of the individual "soul" aided by the son's efforts to get better placements for his parents and grand-parents etc. thus it is the Son who by his actions serves to liberate each soul form one level to another.

Those who do not have sons or worse still who do not perform their ordained duties to their departed elders, they have no choice other than top move on their own and if they haven't done necessary good deeds while living will fall into the vaitaraNI or go into pitR6iloka without special passes. They will be helped by the random sarva pitR^i tarpanams or gayA shraaddha piNDams.