What is a topic sentence a sentence that contains the main idea

That's right. A topic sentence is a sentence that expresses the main idea of the paragraph where it's stated. In an article, there could be a lot of topic sentences. Every paragra[h should ideally have one topic sentence and main idea. The entirety of the article has a separate main idea itself. However, for coherence and cohesion purposes, each paragraph should revolve around one gist.