Very simple to understand. Let me take the word “Apple” as an example. Apple has two syllables “AP” + “PLE” (pronounced Ap-pul with the emphasis on the Ap)
It is confusing because some words begin or end with double letters where the rule of pronouncing the letter twice does not apply, such as “full” or the name “Aaron”.
In those cases the pronouncer would be a slightly stronger emphasis on the double letters.
Hope that helps.
Thank you for being kind.