“Body mass index (BMI) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m2, resulting from mass in kilograms and height in metres.”*
It’s purpose is basically used to determine if we are underweight <20, normal weight between 20 and 25 or overweight above 25. It is a rule of thumb and does not take into consideration other factors such as how much of the weight is fat or structural such as a thin boned or heavy boned body or muscle mass which can siginifcantly impact the interpretation of the results. Someone who is very musclular will have a high BMI and it can be considered healthy for example. It is good to know ones BMI as it can be combined with other measurements such as the Body Fat Percentage BFP to determine a more accurate and personalised weight condition and to determine nutrtional and dietary plans. At the below link you can find more detailed information…
*https://en.wikipedia.org/wiki/Body_mass_index