Scarlet Tie: please explain

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Judah says to Onan, take your brother’s widowed wife
Raise some offspring to his memory and so redeem his life

Onan, no redeemer he, spilt his seed upon the ground
Poor chaste widow, Tamar, no kin redeemer  found.

Judah says to Tamar, “Let me lie with you this day
And whatever is the harlot’s hire to you I’ll gladly pay”
Says Tamar, “Give me staff and ring, and cord around your waist
That Father’s seed, and scarlet tie, the son’s name may replace
Says the midwife to chaste Tamar, “Twins do strive within your womb
Tie scarlet cord around the arm that first does leave that tomb.”

Says Perez’ spawn to Rahab, “If you fain would save your life
Hang scarlet cord from window sill when all around is strife
Then like Israel in Egypt you must tarry in that room
Thus your life shall be redeemed, from Jericho’s sad doom
Says Rahab to her saviour, “Though a harlot’s life I lead
I will marry you tomorrow and my womb shall bear your seed

Says Rahab’s son to Moabite, awakened from a dream
“Another has the kinsman’s right tis he who must redeem
But should he not redeem your life then you shall be my spouse
And you shall be the mother of a righteous royal house
And though that house in time may fall, its stump shall still take root
And from that stump shall come a branch that bears most righteous fruit
Not in Athens, Alexandria, Jerusalem or Rome

Says the angel to the heavenly throng, assembled round the throne
Who is worthy to unseal the course of history, yet unknown
Put to flight all Satan’s hordes in Zion’s troubled hour
And reign in wisdom, might and glory, 
honour, wealth and power.

Not in Athens, Alexandria, Vienna or Rome, 
But on Zion’s holy mountain, on David’s righteous
In spirit and truth, and the hearts that bear the fruit
The essence of those righteous minds 
The scarlet tie that binds

2.0 Redemption: Give Life by Impregnating a Widow

2.1 Onan Refuses to Redeem his Dead Brother’s Life

2.2 Judah Impregnates his Daughter-in-Law

2.3 God’s Promise to Abraham Passes Through Sons

2.4 Ruth Asks Boaz to Redeem Her Husband’s Life

2.5 David Inherits God’s Promises to Abraham