What is a allusion from a poem?

In the fifth stanza of the poem, the speaker speaks of "[his] sorrow must be laid/On your head like a crown."

This is a direct allusion to Jesus Christ's crown of thorns which he wore during his crucifixion. Jesus died on the cross to pay for everyone's sins (sorrows etc.) thus connecting his crown of thorns to Cullen's allusion to his poem. This allusion serves to establish that everyone suffers if one suffers, if one is in pain, then everyone is in pain, etc.