It's one of the 200 factors that affects how well your website will rank on Google. "Nofollow" tells Google not to take the link into consideration and it has no effect. A "dofollow" link will notify Google that you're endorsing the site you're linking to, and hence it passes a "link juice" to the other site. For instance, if a quality site like Forbes links to your website, it should increase your website's authority as a whole. As your website's authority increases over the long run, it should be easier for your articles to rank well on Google.