We have no idea what selah means. It occurs three times in today's daily lectionary Psalm: after verses 4, 5, and 7. It might be like when we add "Amen" after prayers and other parts of our liturgy in the bulletin--some sort of corporate response to the words just said in the context of a worshiping community. It might be the equivalent of a horizontal line across the page marking a division. It might be instructions for musicians like a breathing mark or a caesura in music. Some English versions of the Bible just leave it out, like the NIV. Most that include it leave it untranslated, like the King James and the NRSV. I like one edition of the Amplified Bible that writes "Selah [pause and calmly think about that]!"
So when I read Psalm 32 and see those three Selah's, I think that those are places where we should pay special attention and thought. In Psalm 32, the opening stanza in the voice of what I'll call a wisdom-narrator saying, "Happy are those . . . ," in this case those whose sin is forgiven and covered, etc. The next three stanzas shift the voice to the person this wisdom-narrator was talking about, and at the end of each of these stanzas is one of the Selah's. It's almost like a little play. The wisdom narrator introduces the the idea, then other actors enact it. Maybe Selah is stage direction: wait here for the audience reaction, or move to your next mark.
The three places where we are to "pause and calmly think about that," are in order:
Before I confessed my sins, God's hand was heavy on me, I was miserable, even my body was affected and wasted away
When I confessed my sins, God forgave.
God protects us, preserves us from trouble, surrounds us "with glad cries of deliverance."
Happy, indeed are those who's sins are forgiven.
After the little play is finished, God speaks. God says, "I will instruct you," which sounds like a caring teacher to me, and "don't be a stubborn mule," which sounds like a frustrated teacher to me. God cares for us and shows us the way we should go and wishes that we would stay near to God and walk in God's ways.
In the final stanza, the wisdom-narrator returns. The wicked are tormented, "but stead fast love surrounds those who trust in the Lord," as we've just seen in the little play, and as we've just heard God basically say out of God's own mouth. The final verse is the only real imperative. You'd think that the imperative would be "repent," or "confess," since that's how the little morality play goes: wicked suffer, and when you confess your sins the suffering is relieved. but that's not the final imperative. The wisdom-narrator doesn't say "Be righteous, be upright in heart." The wisdom-narrator assumes you're righteous and upright in heart. The final imperative is "Be glad in the Lord, rejoice, and shout for joy."