Okay kiddo, a salsabil is like a magic water fountain. Have you ever seen a fountain before? It's like a big bowl of water that shoots out water high up into the air and it looks very pretty. A salsabil is like a special fountain that is in a place called Paradise. Paradise is where good people go after they die and it's a really special and happy place.
Now, in Paradise, the salsabil is a very special water fountain because the water in it tastes really, really good. It's like the best drink you've ever had and it never runs out! That means you can have as much of it as you want, forever and ever. The Prophet Muhammad (peace be upon him) once said that whoever drinks from the salsabil will never be thirsty again, isn't that amazing?
So basically, a salsabil is a fancy water fountain in Paradise that never runs out of delicious water and it's a really special thing that people really look forward to having when they go to Paradise.