Planks primarily target your core muscles, including your abdomen, but they don't directly burn fat from your waist.
While planks strengthen your core and improve posture, they don't create a calorie deficit, which is necessary for fat loss. To reduce waist size, you need to combine planks with a balanced diet and regular cardiovascular exercise.
Planks can help define your waistline by strengthening the muscles underneath, but they won't directly shrink your waist.
Think of planks as a tool to improve overall fitness and core strength, not a magic solution for spot reduction.