Central ideas are the most IMPORTANT ideas in a text.
They are the central, unifying and definite themes or ideas of an article or story. They cover all the aspects that are needed in order to create a main idea that is coherent. Without a central idea the text would not be coherent as it could addres different topics that have nothing in common, making it senseless. In an essay the central ideas are supported by the arguments, for example a central idea could be: Bees are important for our planet. In that case supporting ideas can be: They pollinate flowers. They make honey.
As you can see they are the spine and base of the text.
