a link that contains the full URL path in the source code. e.g. http://www.yoursite.com/green-product.html, as opposed to a link that points to /green-product.html.  In terms of appearing in the actual page code, an absolute link would appear as:

<a href="http://www.domain.com/pagename.html"></a> 

Whereas a relevant link would look like:

<a href="pagename.html"></a>

Search engines have an easier time following absolute links than relative links.