在天愿作比翼鸟在地愿做连理枝
[zài tiān yuàn zuò bĭ yì niăo zài dì yuàn zuò lián lĭ zhī]
This comes from classical Chinese literature, meaning 'as birds flying together in the sky or trees growing entwined on the ground, we'll stay as devoted lovers.' This symbolizes a wish for undying love that lasts eternally, through various lives or forms.