Are you talking about your book's ranking? As far as I know, tags don't affect that? 
The tags there for readers to find items on Amazon. If they do a search for a particular term, and your book has been tagged as that term, it is likely to appear higher in those search results.
It is also supposed to help readers organise their own searches. See more here:
http://www.amazon.com/gp/help/customer/display.html?nodeId=16238571
There was a thread on here recently in which somebody said Amazon is planning to discontinue tags -- cant find it right now! But it seemed more than just a rumor, people were quoting emails from Amazon.

The tags there for readers to find items on Amazon. If they do a search for a particular term, and your book has been tagged as that term, it is likely to appear higher in those search results.
It is also supposed to help readers organise their own searches. See more here:
http://www.amazon.com/gp/help/customer/display.html?nodeId=16238571
There was a thread on here recently in which somebody said Amazon is planning to discontinue tags -- cant find it right now! But it seemed more than just a rumor, people were quoting emails from Amazon.