Some paragraphs have a topic sentence that tells you the main point of that paragraph. The rest of the paragraph explains the topic sentence more fully.
Look at this paragraph from a text about dealing with debt. Which sentence is the topic sentence?
Don't look for quick fixes. Using a debt management service may seem an easy solution, but don't forget that you have to pay for their services, and this money would be better spent paying off your debts. Don't even consider borrowing more money from a loan company. You will be paying a massive rate of interest and just exchanging one debt for another.