You'll come across special trees on your adventures in Pokémon Brilliant Diamond and Shining Pearl, which can help you catch some rare Pokémon if you slather them with Honey.
How does Honey work?
When you arrive in Floaroma Town for the first time, you'll learn about Honey Trees. These are dull brown trees, easily spotted amongst the seas of green.
If you inspect the tree, you'll be asked if you want to slather it with Honey. Doing so will attract wild Pokémon.
You can purchase Honey from the old man in Floaroma Meadow, after you've saved him from the Team Galactic goons as part of the story. You'll also find it out on the various Routes in Sinnoh.
Come back after six hours have passed, and inspect the tree to instigate a battle.
You'll know if a Pokémon is there, as the tree will shake. The more shakes, the more chance of a rare Pokémon.
What Pokémon can be found in Honey Trees?
Note that a Pokémon's spawn rate in a Honey Tree depends on whether or not a Munchlax is present in that tree.
There's a 1% chance you'll encounter a Munchlax in four specific trees determined by your Trainer ID and Secret ID when you begin your adventure.
These four trees never change, so when you encounter one, remember that tree for the future.
This website may help you determine which trees are relevant to you, although it is unlcear whether this has been updated for Brilliant Diamond and Shining Pearl.
There's also a chance that nothing appears, and you'll need to slather the tree once more.
All Sinnoh Honey Tree Locations
Note: To reach the Fuego Ironworks, you must use Surf to the left of the small bridge on Route 205, just before you enter Floaroma Town from the west.