The title is an HTML attribute that does not actually show up on your pages, but is displayed at the top of the browser window. By default, most blogging software makes the title the same as the post title or headline.
This may be one of the most important elements to consider altering. Many times your headline for a post benefits from being catchy or even intriguing to attract readers from Twitter, but that may not make the best title for people searching.
You can change default settings in the code or you can use one of the many SEO plugins designed to give you the flexibility to alter the important the elements I address in this post. I use a plugin from Yoast called WordPress SEO.