The word Jesus uses in Greek is γέεννα (Gehenna), which actually means “The Valley of the Son of Hinnom”. An over simplified description of Gehenna would be that it was the garbage dump outside of Jerusalem; this was the place where both garbage and dead bodies would be discarded and consumed by a fire that was likely always burning. The location goes all the way back to the book of Joshua, and was a place where bad things happened– child sacrifice, bodies were cremated, etc. Basically, imagine a dump where garbage is burned add into that the vision of burning bodies and a historical connotation of child sacrifice, and you’ll see that it wasn’t a very desirable place. However, it was a very ​literal​ place and the original audience of Jesus would have understood it as such. They would not have heard the word Gahenna and thought of our concept of hell– they would have realized Jesus was talking about an actual place outside the city.