The sentences in a paragraph should provide details that explain or develop the main idea of the paragraph. These are called supporting details.
Think about supporting details in terms of a job interview. It is not enough to say, “I feel I am a qualified candidate for this job.” You have to back up that statement with reasons why you are qualified and include details such as your training, special skills, and work experience.
The most effective details are those that include facts (information that can be proven true), logical reasons, and examples of things that have actually happened rather than personal opinions or beliefs.
Make sure that sentences with “I believe…” or “Many people think that…” are followed up with evidence that proves those statements to be true.