It allows advanced PHP replacements for the "description" meta tag.
Note: You may want to look at Custom Tokens to define your own tokens.
Usage
Enable the module
Go to Administer -> Content types
Edit any content type and look for the Meta tags node type field set.
Configure any default keywords, description and robots for that content type.
Note: For each content type, if you fill in keywords, description or robots, those will be used in place of the nodewords global defaults, so bear in mind to include them in the content type meta tag node type fields if you need them as well.
Note: if you leave keywords and description empty, the nodewords global default will be used. However, for robots, you must specify a default, otherwise no robots will be displayed on that content type.