On the internet a nofollow link is used as a way to avoid giving your "vote" to a link on your website to a third party.
Think of it like offering your friend a list of painters in your area except with the caveat that you haven't used any of them before so you aren't giving them an official endorsement.
Why is this important you might ask, because Google says it is. OK, that isn't the only reason but it is certainly an important one. Google and the other major search engines have been working for years to sort through spam sites with paid links with sites with real value in their endorsements. In 2005 the nofollow link first came on the stage but was not yet widely used but over the years it has become more widely used in the web development community.
Why in 2017 and I posting about it again, because more non-technical people now manage their own websites using Content Management Systems (CMS). Whether Drupal or WordPress or a third-party service a lot of these people aren't aware of the little tweaks needed that aren't part of the user interface they might be using.
If you are interested in learning more contact out team to schedule a training session on how to add nofollow links to your website.