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In plain English, the quotation, referring to Israel, speaks of their blessing if they would only be obedient to God. They would be the head of the nations instead of the "tail" or the lowliest of nations. Israel's subsequent history proves that they made the wrong decision. This will soon be rectified permanently when Jesus returns.
Deuteronomy 28:13, And the LORD will make you the head and not the tail, and you shall only go up and not down, if you obey the commandments of the LORD your God, which I command you today, being careful to do them...
I hope you have found this helpful.