The Second Coming of Jesus Christ is imminent (that is it could happen at any time). Here Paul uses the imagery of "A thief in the night" like "labor pains coming on a pregnant woman" -- images of sudden changes in circumstances. We don't know when Jesus will return but we know it could be anytime. However, for the believer this is not a time of terror or fear but the realization of our hope. He then admonishes the Thessalonians to stay awake, stay sober, and live in the fullness of the daylight.
Paul concludes with his usual words of encouragement: respect those in charge over you, be at peace among yourselves, admonish idlers, help the weak, don't retaliate, rejoice, pray, give thanks, do not quench the spirit, or despise the prophets, hold fast to good, abstain from evil! He then offers a blessing and asks for prayer.
The point of the letter: be comforted in knowing that your faith will not prevent those who are faithful and did not live to see the second advent of Christ from attaining glory. Be faithful to what you were taught.