Help understanding song titled 天嫁
My take to this is different. 

'桌玛' in Tibetan is like 'goddess'.

We can seize the meaning of '天嫁' from this two lines, 那是天上的桌玛, 骑着白驹要远嫁. It means the goddess from haven will go to a marriage （on the earth）faraway from her home(haven) by riding a white horse. 

So, '天嫁' means the marriage of the goddess from the heaven. It seems that she suffered lots of hardship from there according to the song.