Nofollow plugins

I’ve been reading about “nofollow” plugins and I still don’t know what they would be used for. If I create a post and attach a nofollow tag to it, that would mean that search engines wouldn’t follow it. Follow it where? Or would it not index it. What good is that if I’m trying to rank for my blog?

If someone can enlighten me on what nofollow links are used for I’d appreciate that.