Defog is one of the TMs you can find in Pokemon Brilliant Diamond and Shining Pearl. This TM is also one of the hidden machines the Poketch can use to remove obstacles in your path as you traverse the map. Specifically, Defog will remove fog in areas with dense fog. It's great for areas like Route 210. This is where you can find the TM.
Before you can get the TM, you will first need to progress enough in the game. On your journey, you will encounter Veilstone City. After defeating the Gym Leader here, you will be directed to Pastoria City. Take the path south of Pietravelo and be sure to rest before you leave as there are plenty of trainers to fight along the way.
Once you reach Pastoria City, head to the Great Marsh. The building is located in the northern part of the city and will cost you $ 500 to enter. After entering the Great Marsh, go right to find a green-haired NPC. He talks to them and they will give you the TM Defog.