You need to read it.
What is a backlink?
The backlink is one kind of link it's mostly used to get more ranking on google. Google always ranks any post by seeing how many backlinks have this post or website. For example, you have two sites now you give your one site link to another now google spider patch this URL and run your spider on your other site and your second site index faster.
What is a do-follow backlink?
A do-follow backlink is the best backlink for rank your site if anyone gives you a do-follow backlink then when google spider comes to patch the site the spider also patches your site so your site is patch faster than before. Many people know Google's old name is backrub (google work by backlinks this why the owner of google take the name backrub then he changed the name and take google).
What is a no-follow backlink?
No-follow not so good for site ranking because 99% of Google bots don't give patch your site.
How to give do-follow or no-follow links?
When you give any link to other sites it's always set on a do-follow link but if you want to give a no-follow link just add it before your link (rel="nofollow") after adding this code your backlink is converted to no-follow.