Those are tumblr tags. On Tumblr, if you want to respond to someone's post, it goes onto your blog, which means your followers or people who visit your blog will see your response. But if you're a regular user with a small following, not many people will see what you say. So it's common practice for smaller users to respond to big posts like this in the tags, so that more people will see them. It's kind of an antiquated thing that has persisted since the website first started

Tumblr

It's just users using the tags to comment. It's something of an old Tumblr tradition, from before replies were added. Doing things that way means that you could reblog something to your profile, and add commentary about it without visibly changing anything meaningful, since a regular reblog would tack the post onto the end, which you might not want, if you want the post to be seen as it was.