What does Colossians 1:21 mean?

He is the head of the body, the assembly, who is the beginning, the firstborn from the dead; that in all things he might have the preeminence.

For all the fullness was pleased to dwell in him;

and through him to reconcile all things to himself, by him, whether things on the earth, or things in the heavens, having made peace through the blood of his cross.

yet now he has reconciled in the body of his flesh through death, to present you holy and without defect and blameless before him,

if it is so that you continue in the faith, grounded and steadfast, and not moved away from the hope of the Good News which you heard, which is being proclaimed in all creation under heaven; of which I, Paul, was made a servant.

Now I rejoice in my sufferings for your sake, and fill up on my part that which is lacking of the afflictions of Christ in my flesh for his body’s sake, which is the assembly;