Tohjo Falls is a location in between the Johto and Kanto regions. After obtaining the eight Johto League badges, trainers pass through here to enter the Kanto region.
It is said to be a trainer's first step into the Kanto region. This statement is true, due to the fact that is the first place in Kanto, one can access after leaving Johto.
If you look on the Town Map, you can see that you're in Kanto now.
In HeartGold and SoulSilver, the Celebi Event revealed that Giovanni, the ex-leader of Team Rocket, hid here during the Goldenrod Radio Tower takeover and evidence of this is a new room with a broken radio and breakable rocks.
The "Tohjo" part in Tohjo Falls is combination of Kanto and Johto.
Tohjo Falls in Gold and Silver
A picture of the falls in HG/SS