Joanna Penn swears by this. Write stuff that has to do with your writing. Give it away free and people will be able to see what your writing style is like and they will buy your books.
What is Content Writing for authors? Well, it is the meat of your website. If someone is wandering your site this is what will make them stay. It is the content. The more you have the greater the chance people will stick around longer and like any retailer will tell you, the longer someone is in your store the better the chance they will buy something. Why do you think grocery stores make you wander the aisles to find anything. For writers that content can be very diverse. In fact it should be diverse. Many authors think they should write about writing. Well, you can but almost everyone does. I put my advice on another site as I found it clashed with my author brand. Make sure whatever you write fits with the image you want to give people.
What to write? Whatever makes you excited. If you like dragons, write about dragons. If you love cats then write about cats. Because be honest, what you love will end up in your books. You see Google will send people your way when they look for stuff. So the more you write the more chance Google will send them your way.
Suggestions for writers (other ideas for what writers should be writing about)
Your woohoo moments
What you are doing
What you found when you were researching
Interviews with characters
Lesson plans to go with your books
Whatever you write, it needs to be out there. You can also do this on a Facebook page. Use your imagination. Also, if you are brave try other forms like Youtube.