I don't even know what they mean.
MB how could you understand so much about puters and you run your own website and you got in early even when whole lotsa folks didn't understand puters but you don't understand hashtags. You need a Young Person to hang with!
Fwiw, my understanding of hash tags on IG is: Let's say you want an IG account for your dog, you want the world to see how cool and cute he is, his name is Fido so you might create an IG account for him called @Fido. Btw, when you are reading "@Fido" you could just think it as "Fido" to keep it sensible, but the "@" symbol means the next word following is the name of an IG account. OK, when you post a pic of Fido which your IG followers will see cuz they follow Fido cuz you told your friends you made an IG account for Fido so check it out, you might also post some hashtags in the caption so that people on IG who never ever heard of you nor Fido, but who might be interested in dogs or similar stuff, could see Fido if they clicked on your hashtag links that you made, like maybe #cutedogs or #cute_dogs or #furryfriend, etc, etc, peeps that wanna check out cute dogs on IG would type in #cutedogs and up would come Fido's pic and post, along with all the other peeps who posted their pics to #cutedogs
Or, let's say you designed some awesome dog collars and you posted a pic of Fido wearing the collar for fun as well as advertising, you could make a hash tag #dog_collars or something similar, peeps who typed in #dog_collars would see your pic of Fido and his collar and if they wanted to learn more they would click the link @Fido and see your posts about Fido and you'd have info and/or a link to your site where they could buy the dog collars you are selling.
Ha, hard to believe I might actually have all the above right, so apologies now for any inaccuracies, but that is my understanding of hashtags on IG. Don't know about twitter.