What's Lacking "in Regards to Christ’s Afflictions"?

What does Paul mean when he talks here in Colossians about that “which is lacking in regards to Christ’s afflictions?”  Is there something missing that we should know about?  Even worse, is there something that you and I need to do to 'finish the job' of salvation?  No, thankfully, not even close.  So what is he telling us here in this passage?