This is basically a very poetic way to say; "Blessed is the man who is saved by God's Grace."
Paul is teaching the Romans about the Grace of God. In chapters 4-8, Paul uses Abraham as an example to show that individuals were not justified through obedience to the law of Moses—they were justified through faith in God’s promises. Since Abraham lived centuries before the law of Moses was given, he was an ideal example. He then segues into a quote from David from Psalms:
Even as David also describeth the blessedness of the man, unto whom
God imputeth righteousness without works, Saying, "Blessed are they whose iniquities are forgiven, and whose
sins are covered. Blessed is the man to whom the Lord will not impute sin." (Romans 4:6-8 emphasis added)
Compare with David's words in Psalms:
"Blessed is he whose transgression is forgiven, whose sin is covered.
Blessed is the man unto whom the Lord imputeth not iniquity, and in
whose spirit there is no guile." (Psalm 32:1-2 emphasis added)
Christ suffered for the sins of all humanity, it is only through his grace that we are saved, that is what these verses are trying to communicate. By God's grace all men may have a portion of Christ's righteousness (God imputeth righteousness) and be cleansed of all our sins (the Lord imputeth not iniquity)
To attribute (righteousness, guilt, sin, iniquity, etc.) to a person or
persons vicariously; ascribe as derived from another.`
When Christ suffered the pains of the atonement he took upon him all the sins of the world, he bore the burden of sin for every single person on earth and suffered terrible anguish in our behalf. Figuratively speaking, in that act, he sinned–vicariously–through us. He felt the pains of remorse that all of us would feel because of our sins, he gained a perfect knowledge of the trials and tribulations of every one of God's children. He took all of our sins upon himself, and if we but follow him, then he will forgive us of every one of them.