Blog commenting mean live comments on other blog. You can also to get backlinks from blog commenting.
From the SEO point of view, blog commenting helps you to make updated your blog as well As make visible in front of the search engine as well.
small question on the topic ... assume the site has been selling soft toys, blogs on the topic of soft toys, almost not, whether a bad sign for the search engines if the links on this site are in the comments to a whole other topic not related to soft toys?