Guest posting is a type of contribution to another blog, and when done correctly, it means that you write a unique blog post and publish it on another blog. That post will solely belong to the blog owner and can't be republished anywhere else.
There are actually many people who refuse to do guest posting on others blogs because of this one reason, "When I write a unique and awesome article, why don't I write and publish the blog post on my own blog?", this question has been asked and answered thousands of times, but still there are people refuse to submit guest posts on other blogs.