Perhaps you've heard that in order to lose weight, need to do more reps with lighter weights. (fitnessandpower.com)
Another example would be to use a dumbbell between your feet for a weighted pull - up and then dropping the dumbbell and doing more reps with just bodyweight. (t-nation.com)
The absolute worst thing you can do is start purposely using lighter weights for more reps because you think it will burn fat. (acaloriecounter.com)