The All In One SEO Pack plugin should be considered mandatory for all WordPress blogs. If you are putting your thoughts out on the Internet, you want them to be found and read be interested people. Unless they find out about your blog by word-of-mouth (a topic for another post), they need to be able to find your posting when they search the web.
The search engines (Google, Yahoo, Bing, etc.) rely on many factors when determining what pages they return and in what order. These factors can generally be separated into two types, on-page and off-page SEO (Search Engine Optomization). The off-page factors deal primarily with links on other pages and how they link back to your posts.
This plugin helps you with the on-page SEO piece. The on-page SEO factors that WordPress does not handle directly for you concern the title, description and keywords for your post. The title you give to your post on the Add New Post page should include the major keywords that someone might use to find your post. This plugin adds the section shown below to the Add New Post page.
The title section here is what will be displayed at the top of the browser (or on the tab of a tabbed browser). It is also the title that the search engine will return as part of its results page. The description is what will be displayed below the title on the results page. You want to include your major keywords in both the title and the description in a reader friendly way. The search engines will look at them to see if they apply to what the searcher asked for. But it is not all about the search engines. Once your post is displayed to them, it is up to your title and description to catch their eye enough for them to click on the link and come to see the rest of your post.
The last edit box on the form is for your keywords. This is all about the search engines here. Put as many keywords here (separated by commas) as you can think of that are applicable to your post. Try not to repeat yourself on the keywords or add keywords that do not appear in your post. The search engines may actually count that against you!
Finally the plugin does give you a checkbox to disable the plugin for that specific post or page. You would use this if you have a post or page that you did not want the search engines to return as a result.