2 Thessalonians 3:10

"For even when we were with you, we gave you this rule: 'If a man will not work, he shall not eat.'" NIV translation

At first glance, these words may seem kind of harsh but the simple fact of the matter is that all men must work for their food. That was a curse that came through Adam's disobedience (see Genesis 3:17). You see, before the fall, Adam and Eve had all that they needed as they were in the very presence of God. 

With the fall and the banishment from Eden, came the need to work for their food. There is coming a time when this will no longer be necessary (see Revelation 22) but, for now, we must work for our food.

2 Thessalonians 3:11 & 12

"We hear that some among you are idle. They are not busy; they are busybodies. Such people we command and urge in the Lord Jesus Christ to settle down and earn the bread they eat." NIV translation

There were some in the body of believers that just hung on and survived by the handouts instead of working. As we see here, they spent a lot of time in other people's business and in talking about others instead of getting a job to buy their food. Paul gave them a strong command "to settle down and earn the bread they eat". The same thing is happening even more today as there are so many that do not want to work but want to sit around and gossip until the Lord comes back. That is not what we are called to do as the giving out of the food should be a tool to reach the lost and not a tool to empower people to sit around and gossip.

2 Thessalonians 3:13

"And as for you, brothers, never tire of doing what is right." NIV translation

Now, Paul urges them to continue to work and share the good news of Jesus Christ. He is not saying that they shouldn't get physically and mentally tired as that is going to happen when we are working. What he is saying is that we do not stop doing the right thing because we are tired and it is easier to do the wrong things.