Say you post a link to your hub in a comment box on a blog. If the blog host has set it up so that the links can be followed by search engines then your hub will get credited with a backlink - basically the search engine will see it as a yes vote to your hub. This is a Dofollow link and is worthwhile.
If the blog is set up so that links are no follow this instructs the search engine bot not to follow the link to your hub, so your hub doesn't get a yes vote. It doesn't get a vote at all from nofollow links, so is worthless in terms of link building.