Blog commenting was a very popular link building tactic where you find a blog article that is relevant to your niche and leave a comment with a link.
Nowadays this tactic is cause for debate. Since it was over abused and people massively spammed blog articles leaving links and irrelevant comments everywhere, comment sections now apply a nofollow to links. Google also put a stop to this practice by penalizing links from irrelevant websites and by reducing the link equity passed on by pages that contain massive amounts of links.
As you can see, this tactic can hurt you more than help you. That being said, there are still cases where this is appropriate. Leaving a valuable comment in a blog article can help you build relationships in your niche and result in guest posting opportunities. You might not get link equity, but what you can expect is traffic increase.