An allusion is a literary device that stimulates ideas, associations, and extra information in the reader's mind with only a word or two. Allusion means 'reference'. It relies on the reader being able to understand the allusion and being familiar with all of the meaning hidden behind the words.

Here's an example:
"As the cave's roof collapsed, he was swallowed up in the dust like Jonah, and only his frantic scrabbling behind a wall of rock indicated that there was anyone still alive".

Here is an example from John Donne's poetry, Holy Sonnet IV
HOLY SONNETS.
IV
O, my black soul, now thou art summoned
By sickness, Death's herald and champion ;
Thou'rt like a pilgrim, which abroad hath done
Treason, and durst not turn to whence he's fled ;
Or like a thief, which till death's doom be read,
Wisheth himself deliver'd from prison,
But damn'd and haled to execution,
Wisheth that still he might be imprisoned.
Yet grace, if thou repent, thou canst not lack ;
But who shall give thee that grace to begin ?
O, make thyself with holy mourning black,
And red with blushing, as thou art with sin ;
Or wash thee in Christ's blood, which hath this might,
That being red, it dyes red souls to white.

To wash in Christ's blood refers to cleanse a person from the inside out and make he/she a new creation, making he/she adopted children of God and giving us eternal life in Heaven with Him.