What is a reblog?
I'd call them trailers for blog posts.
Let's say you write a blog post. So, I choose a short snippet of that post (usually the first lines) then add a link to your post. I put this on my blog.
This way, over 135, 000 people get to see this short preview. Those who want to read more, click on the link and go to your blog.
It's a way to gain exposure.