"We have confidence in the Lord that you are doing and will continue to do the things we command." NIV translation
Paul was confident that God would cause to grow the seeds of the Word that were planted in the believers at Thessalonica. He is also confident that the Holy Spirit will convict their hearts of the need to not only hear and study the Word but to put it into practice. That is the same thing that James was talking about, in James 1:22-25 when he spoke about the man and the mirror.
"In the name of the Lord Jesus Christ, we command you, brothers, to keep away from every brother who is idle and does not live according to the teaching you received from us." NIV translation
Paul now warns the believers to stay away from those who are not willing to work. In Genesis 3, we see that, for man, the result of sin was that we are required to work to obtain food. That was not so before Adam sinned and will not be so in the new heaven and new earth but, for now, that is what we as men must do. Today, it seems like men want to sit around with their hands out and let a government or someone else take care of them and their family's needs but this is not of God.
"For you yourselves know how you ought to follow our example. We were not idle when we were with you, nor did we eat anyone's food without paying for it." NIV translation
Paul reminds them that he not only shared the gospel with words but also with his actions. He reminds them of the fact that he did not consider it his job to share the gospel but he used his job as a tentmaker to support his sharing of the gospel. He could have taught the Word for a couple of hours each week or even each day and sat back to collect a tithe but he did not.