What does Google Seed Sites mean?
In order for Google to rank quality sites higher (and remove pointless link directories and content farms) it keeps a list of websites known as seed sites.
These are websites that it wholeheartedly trusts will not engage in any black hat, or even grey hat, SEO.
The list is not in the public domain but some obvious ones would be Wikipedia, many mainstream news websites (e.g. cnn.com), hand edited directories (e.g. Yellow Pages), etc.
When looking at other websites one
of the factors Google takes into account is how many links away it is from a Seed Site.
So if Website A has a link from Wikipedia it is one link away from a Seed Site and that's a positive result. If Wikipedia links to Website B which then links to Website A this is still positive but does not have the same strength.
Many professional link builders will spend a great deal of time trying to secure a link on a Seed Site (or what they think is or will become a Seed Site) because of how powerful this can be in Google's ranking decisions.
Note - it is perfectly possible that other major search engines are also using this system