What are clean urls and why do they help search engine listings?

Clean URL’s generally appear as legible text in a web site address.
For example you may have seen this:
http://www.mydomain.com?cat=2&prod=45

The link above is not clean, it would be better to see.
http://www.mydomain.com?mens-clothing/grey-trousers

The link above is an example of a clean url. It has taken the values needed to display the page and changed them into english. This can now be indexed and will help with your search engine listings.