It topic sentence the main idea or not?
Main idea refers to a thesis statement, even though thesis statement is an actual sentence, which appears at the end of the introduction, while the main idea is implicit, that is something your whole work implies. Topic sentence opens a paragraph and introduces the topic or idea you cover in it.
Topic sentences should support thesis statement. You can easily understand this having formula for effective thesis statement:
Something is A, because B, C, and D.
Para 1: explanation of argument B. Topic sentence: lead-in into the argument B
Para 2: explanation of argument C. Topic sentence: lead-in into the argument C
Para 3: explanation of argument D. Topic sentence: lead-in into the argument D
You may read more about thesis statements and how they should be supported in an essay here