A hot fudge sundae is just another variation of a sundae - except this one is both hot and cold. I have to admit, dishes that combine the two temperature extremes of hot and cold are always intriguing. They cause my brain to lock into a weird loop of "It's hot...it's cold...it's hot...cold.." which leaves me unable to function until my brain finally goes, "I don't care. It's good."
Anyhow, if you're curious, there are actually tutorials on how to make a hot fudge sundae. Which just seems so silly to me - just take a couple scoops of your favorite ice cream, add some sprinkles or nuts if you want, then pour a hot fudge sauce on top. Add cherry and whipped cream, if you feel like it.
You can buy chocolate fudge topping at the store and heat it up, or you can try making your own hot fudge sundae sauce. If you add espresso to the sauce, you get a little extra kick and you get to call it Italian hot fudge sundae.