Beáti quorum remíssæ sunt iniquitátes: * et quorum tecta sunt peccáta.
The idea of beatitude
Beatus simply means happy, or blessed. It has the same meaning in the (New Testament) beatitudes. In the first verse, it is in the plural; in the second verse, the psalmist continues with the same ideas, bringing it back to the individual.
Why is the psalmist happy? Because his sins (iniquitas=iniquity, sin, or rebellion against God's authority; peccatum=sin, failure, error, going astray) are forgiven or pardoned (the verb is from remittere), 'covered' (tegere) or taken away altogether (the Hebrew suggests something more like 'offend the eye no longer'). The whole thrust of the verse is that sense of a lightening of one's burden experienced (hopefully) when one emerges from the confessional!
Scripture interprets Scripture?
It always important to look at how the New Testament in particular interprets passages from the Old, since the New fulfills and explains the Old. In the case, St Paul quotes this verse in Romans 4, in his discussion on salvation:
Real remission of sin
This passage by St Paul, though, is one of those passages that demonstrate the importance of reading Scripture with the guidance of the Church, for the verses are also used by Luther in his theory of the non-imputation, rather than real forgiveness of sin!
Pope John Paul II puts the text in its orthodox context:
Tomorrow, a look at verse 6 of the psalm, on admitting our faults. In the meantime, enjoy the setting of the first verses of the psalm by the sixteenth century composer, Delalande: