Think of a paragraph as if it were a hamburger. The top bun is your topic sentence. The sentences that come next are your supporting detail. They are like the lettuce, tomato and hamburger patty in the middle. Holding everything together is the bottom bun of the hamburger. It is a concluding sentence. Just like your hamburger would be a mess to eat without a bottom bun to hold it up, your paragraph is a mess if it doesn't have a concluding sentence. Here is an example:
Top Bun/Topic Sentence:The Superbowl is the most important football game in the United States.
Supporting Details/Burger & Lettuce: It is played every January. It is played by the two professional football teams who have the best records in their leagues. Millions of people watch it on tv every year. There have been 31 Superbowls.
Bottom Bun/Concluding Sentence: Almost every football fan hopes to see his or her favorite team play next January in Superbowl 32.