(literary) to show gratitude or repay a debt of kindness; based on a legend where a rescued bird returned with four white rings to reward its savior. Often used in the phrase 衔环结草 xián​huán​jié​cǎo

(literary) to express deep gratitude and a promise to repay a debt of kindness even after death; literally "to tie grass together", referencing a story of a ghost who tripped an enemy to save a benefactor