What does nofollow mean?
When a link is marked "nofollow" it means that none of the link juice
from that page should be given to the website whose link it is.
Most links in blog comments and many on forums are marked this way to dissuade spammers. The message is "you can add a comment here and you can even include a link to your site but it will not help your site's rankings".
So the spammers who trawl the web looking to add pointless comments such as "I agree, great thoughts here, keep it up. Here's another useful site [link to their own website]" decide not to bother and it is one less bit of spam for the blogger or forum owner to deal with.
The exact mark up looks something like this:
<a href="http://yoursite.com" rel="nofollow">Link Text</a>
In this situation search engines like Google ignore it as a valid backlink to your website because that is what the webmaster has requested.