The blog header is the area at the top of your blog that contains your title. If you want to use this blog as an example, "Non-Techie Blogging" is my header. Underneath the blog title is my tagline: "Simple tips and tricks for effective blogging. The tagline is also part of the header.
The header should not be confused with post titles. Each blog post will have its own title. The header is going to be a pretty consistent thing.
If you want to get fancy you can make a custom header. Personally, I prefer having the title of my blog being very clear. I want it to be one of the first things people notice. This helps people remember the blog if they want to recommend it.
I would also highly recommend adding a tagline. Keep the tagline short and precise. It should give new readers a quick one-sentence summary of what your blog is about. I've noticed that a lot of bloggers neglect to add a tagline. Remember: don't make your readers work for information. Make your blog easy to understand and easy to navigate.