The main idea and the topic are two elements of any literary work, which are most times confused to be the same, simply because both give certain information about the central idea of any piece of writing. The topic of any piece of writing prepares a reader as to what he/she should look forward to in a piece of writing.
The first thing that guides a writer while writing is the topic. It is basically what a piece of writing is all about. On the other hand, the main idea of any literary work is the general information that a writer wants to pass across to the readers. The main idea of any piece of writing gives more information about the topic chosen by the writer.
Topics are most times written using simple words, so they don't look too complex to people. When you read a piece of writing, the main idea is communicated by the various sentences used, and this further gives more meaning to the topic. Topics are usually written on the first line of every piece of writing, while the main idea is usually included in the content.