What BMI means
Body Mass Index estimates body fat from height and weight. It's a quick screening number: below 18.5 is underweight, 18.5–24.9 healthy, 25–29.9 overweight, and 30+ obese. BMI doesn't distinguish muscle from fat, so very muscular people can read as overweight without being unhealthy. Treat it as a starting point, not a verdict.
If your BMI number is higher than expected and you want a structured plan, the book Bigger Leaner Stronger is a popular, evidence-based starting point.