Question

explain how images and captions can help understand the main idea

Answer 1

This could have several different answers.
Here's an example.
In your science book, you may have a picture of a hornets nest.
In another science book, you may not have a picture.
If you're learning about something, sometimes it helps to have a picture and a caption to go along with it, so you can have a mental image of what youre learning.