what the heck is a hashtag?

A hashtag is a thing used on Twitter to categorize posts. What you do is, you type the number/pound sign, this thing #, and then a topic of your post. So if I posted a picture of a dog, I could hashtag the post with #dogs. That way, if you wanted to find posts related to dogs, you could search on Twitter for #dogs and find all dog-related posts. Likewise, you can hashtag things for any other subject, #politics, #currentevents, #basketball, and so on.

Alternatively, people add hashtags into comments outside of Twitter for satirical or sarcastic purposes.