Usually the web masters pay little attention to the configuration of links on a website that he develops whether the links will later be DoFollow or NoFollow.
By default the configuration of a website will be all DoFollow links, and to make it NoFollow special configuration is needed.
DoFollow means more or less follow, so the search engine, if you find a DoFollow link then he will search for it, then crawl it as well.
NoFollow is the opposite, the search engine is instructed not to follow the link in a post on the website.
When to do it?
Better if you configure all of the links on a website not all DoFollow, or not all NoFollow.
Do DoFollow when there is an interaction between the discussion of the article with the target link, later search engines will follow it, explore it in depth, if there is a connection then it is good, but if there is no connection, it might be considered spam. It is considered spam because it only stores links aka brutal advertisements … hehehe …
When there is a link, this will affect pagerank, there will be a page rank share, so that between the web referencing the link can increase its pagerank in the search engine world, because it is a valid website with unique quality content.
While NoFollow, it should be done for example when reviewing something, so there will be a link to an address, so why when reviewing, yes, because originally this was a one-way discussion, so it is unlikely that there will be a return from the linked link. Then search engines can be instructed not to follow the link written.
DoFollow and NoFollow will only be felt in the world of search engines, while people who read a site then click on the link there will be no difference, he can still open the link.
Untuk lebih lengkapnya dapat dibaca di https://support.google.com/webmasters/answer/96569?hl=en