BMI is weight divided by height squared — a quick population-level screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so athletes and very muscular people often read "overweight" despite low body fat, and it may under-read risk in older adults with low muscle mass.
BMR (Basal Metabolic Rate) is roughly how many calories your body burns at complete rest just to function. TDEE (Total Daily Energy Expenditure) multiplies that by an activity factor to estimate your actual daily calorie burn — a common starting point for weight management.
Kids' and teens' bodies change rapidly and differently by age and sex, so pediatric guidelines compare a child's BMI to a percentile curve for their exact age rather than fixed adult cutoffs. A pediatrician is the right source for that assessment.