This is one of those terms that techie people throw around completely oblivious of the fact that non-techie people have no idea what they're talking about. "Oh, these are some things that will really help your SEO." Well, if I don't know what SEO stands for, that whole sentence is kind of null and void.
SEO stands for Search Engine Optimization. It sounds scarier than it really is. If you break down the whole term: "search engine" is what people use to browse the internet. Google or Yahoo! are search engines (there are dozens, but those are big ones). Optimization is just that, you are trying to make whatever it is that you're doing the best that it can be.
So, in this case, SEO means that you are making changes to your blog that will affect how relevant your content is. In layman's terms: you are changing if it comes up on page 1 on Google or page 3,122.
You want to work on your SEO because if you want new people to see your blog, you will need to have your blog come up when people type in keywords on Google or Yahoo!. You don't want the only way to see your blog online to come from literally typing in the exact name of your blog. No one except your relatives will do this.
All of the posts I have listed in my How To Get Blog Traffic series are things that will help your SEO.