I've heard many times from people about reaching your target heart rate to burn fat. While reaching your target heart rate does burn a higher percentage of calories from fat, if your goal is to lose weight, then you need to focus on the number of overall calories that you burn during a workout.
Many cardio machines have target heart rate monitors and will let you know when you are in the "fat burning zone." There is nothing wrong with working within this zone but if you want to drop weight, then remember calories in / calories out. If you burn more calories than you take in, you'll begin to lose weight.
