Dofollow simple means the ability of a website or weblog to be able to link back to other website link that was left on some of its pages by another website commenter over the internet. In most cases if dofollow is enable on blogs, it will allows readers on your website to be able to visit the other website through (link left on your website through comment)the backlink that the commenter leaves on the site.
Commenting can also be liken to dofollow because there is nothing you can do to get backlink to your blog from another website without you having a relationship with the site (exchanging links with other website by commenting on the posts on the blog).
Though, commenting as a human-being is the legal way, and known as the best act of do-following on earth but some internet blogger (internet aggressive marketers) use blog commenting spam-bots and software to comment on blogs in order to get back backlinks instead of offering the readers an insight to the post content, they spammed the system instead. The major purpose of taking part in blog commenting for this type of people is for the sake of getting traffic to their blog at all cost and this is illegal.
Dofollow is good for internet bloggers that wants good online presence in a short possible time but, only if the person can do it legally.