Blog commenting is posting comments on blog posts. There are millions of blog posts updated on millions of blogs everyday and you post your comments under a blog post about that particular post usually with a link back to you own website/blog if you are doing blog commenting to increase your backlinks otherwise you can post your comments without any links if you are perhaps interested in that particular post or have any question regarding it.
You'll want to make sure that your comments don't come off spammy. Blog commenting can be a good idea to gain traffic, but don't just overload other's blogs with your ad content. You'll be blocked and removed faster then you can gain traction to do yourself any good.