The short explanation for Hisss is that it’s a tile laying game where you are building snakes. The heads and tails are all either one color or wild but each segment are two colors. Colors need to match when placing tiles, just like in games like Carcassonne. If you complete a snake, you get that snake and its tiles count as points. Most points wins.
There are three things that made Hisss a more enjoyable experience than Rivers, Roads and Rails, most of them being the game being simpler. There are less than half as many tiles. The connections are simpler, one snake segment as opposed to three kinds of possible paths. And the rules are _much_ better written.
In short, Hisss is a lot more accessible for little minds who don’t have that much patience. Hisss takes the concepts of tile laying and makes them manageable for the young. Which isn’t as easy as it sounds. And having a tighter rule set is so much better.
For adults, it’s not a great game. I’m not even going to call it a good game. Hisss is not one of those kids games that adults can get into. However, it is a game that can keep a child engaged until the end. That might be be damning with faint praise but it’s also true.