When you make or reblog a post, you can add tags at the bottom. Ostensibly these are for searching/categorization, but people often use them to write out responses to posts so that their followers can reblog the it without bringing their comment along (Tumblr just puts all replies into a single extended post so it’s a bit cumbersome to have long comment chains). The tags are visible in the “notes” section of the post, so people can still see it.
When you see a screenshot like this, it likely means that the response was made by someone else and the OP self reblogged it because they thought it was important.
Yep, though there’s certain punctuation that will break them. I think this practice is why even regular Tumblr posts tend to have strange grammar.