As you begin to enter Victory Road, you’ll be given Waterfall ; this HM lets you climb up and down waterfalls.

Clearly, since it’s so late in the game, you’ll have skipped over some areas in the game that you couldn’t access without Waterfall. So once you have Waterfall, you may wish to head back to these locations and access the areas that required it. There aren’t many, so it shouldn’t take too long to cover it all.