Paul was in prison in Rome awaiting trial, but his heart was free. He was cared for by his young friend Timothy and allowed to write letters to the new churches.
This is Paul's usual written greeting. But, unlike many social pleasantries (in England we ask, "How do you do?" but nobody expects an honest answer!) it really means something.
Christians ought to pray for each other. Paul's captivity allowed him even more time, and God used his prayers in the upbuilding of churches and people.
There are three main sources of fulfilment for those who believe in Jesus. These have nothing to do with money, possessions, power, prestige or any achievement!
The gospel is all about restoring sinners, who repent, to fellowship with God and His people. Paul has seen the penitence of Onesimus, and observed his changed life.
Although Paul was in prison because people did not like his faith in Jesus, he knew he had more work to do. What an act of faith to ask Philemon to prepare his guest room!