From Bulbapedia, the community-driven Pokémon encyclopedia.
Ledges (Japanese: 段差 ledges) are a common feature of the geography of the Pokémon world. They can be found on most routes, and have been featured in every core series Pokémon game.
Ledges are not just for decorative purposes; they also serve as one-way obstacles. Players are able to jump over ledges and land on the other side, but may not climb back up. A ledge may be strategically placed on a route for a couple of different reasons.
One purpose the ledge serves is to provide a quick return from a new area once it has been reached. Often this removes the need to walk through tall grass (in one direction only). Ledges may also be used in this manner to provide a quick return back to the start of a cave.
The other main use of a ledge is to prevent a player from entering a new area too early. For example, it is possible to enter the southern end of Route 46 near the beginning of the game, but the northern end cannot be reached until reaching Blackthorn City due to the ledges blocking the path. This area is full of high level Pokémon, and being able to travel there at the start of the game would disrupt the storyline of Pokémon Gold, Silver, Crystal, HeartGold, and SoulSilver.
In Generation III
In the Hoenn Safari Zone and the Jagged Pass, when there is a stepping stone running across a ledge, it is possible to use an Acro Bike to hop "up" the ledge across the stones. This can be achieved by holding down the "B" button to bunny-hop, and then hopping up onto the stepping stone onto the higher ledge.
In Generation VI
Ledges in Route 12 can be jumped over either way by riding a Skiddo from Baa de Mer Ranch. It is not possible to ride a Skiddo anywhere else, however, even if one is in the player's party.