So, having back links from do follow sites such as forums boosts your page's ranking in the search engine result pages.
Posting comments also at high traffic do follow blog sites on topics similar to what you have on your website and adding your targeted keyword in your hyperlink will encourage the blog owner and blog readership to reciprocate and this counts as a vote to your web page.
So, the more you have these links pointing to your site the higher your page will rank in the organic search engine result page.
When the search engine bolts visit your website, what they do is to read its pages and count the incoming links to it and use these to ascertain their rankings.
The Dofollow links they will follow to their destinations, indexed, and include in their search results where the Nofollow links are excluded in the organic search result pages.
It is so because the owners of those website's links use the no follow links that prevents you from receiving their vote.
Do not forget that Dofollow links are great ways of building back links that will improve your website page Rank in the search engines and that Nofollow links are another great way of building back links that gets your website high quality Referrals.
Note that it is up to you to choose which one to use when placing your link.