Let me tell you: It is, at its very essence, your ability to reach your intended audience and influence them to buy your book.
That means it is not a specific thing, although there are specific things that help you do this better than other specific things.
Remember, you can reach your intended audience, but if they aren't buying your stuff, something else is wrong and your author platform isn't as strong as it could be.
If you'd like to learn how to build a strong author platform, check out my video at
https://authorneering.com/platformclass.