Working with TAGS, TITLE TAGS and stuff like that

I’m trying hard to understand how tags work on wordpress in order to improve my google pagerank. My header.php file is currently set up like this:

<meta name="Description" content="xxxxxxxxxxxxxx" />
<meta name="keywords" content="yyyyyyy" />
<meta name="author" content="Fernando Souza" />

x being the current description and y being some keywords I judge right for the site’s content.

Now, my question is: By leaving it like this, will it override all tags from the posts?

Should I remove that piece of code in order to improve google results by having specific tags for specific posts?

Well, as you see I’m pretty confused. Any help here is appreciated! Thanks a lot!