Incidental means secondary in time or importance. If you lose weight because you moved and must walk further to school, the weight loss was incidental to the move.
When something is incidental, it is never the main thing. It's okay to include incidental details as they can add depth to your storytelling, but they are never crucial to the story. A company might pay your main expenses when you travel for business but will probably not reimburse you for the incidental ones. The music you hear between acts of a play is called incidental. It's not crucial to the action, but it makes the waiting easier.