Ideal weight is an estimated body weight that clinical formulas derive from nothing but height and sex. There is no scientifically defined ideal weight, so a range is more honest than a single number.
The common formulas
The four best-known formulas come from clinical practice between the 1960s and 1980s: Hamwi (1964), Devine (1974), Robinson (1983) and Miller (1983). All of them start from a base value at a height of 5 feet (152.4 cm) and add a fixed amount for every inch above that. For men, Devine uses 50.0 kg plus 2.3 kg per inch, for example.
Example: A man who is 1.80 m tall is just under 10.9 inches taller than 5 feet. The four formulas give him about 71.5 kg (Miller) to 77.3 kg (Hamwi), with Devine at about 75 kg. A BMI of 18.5 to 24.9 would correspond to roughly 60 to 81 kg at this height. This is a worked example.
Why it is of little use for lifters
The formulas know nothing about muscle mass, build or body fat. If you have been lifting for years, you often weigh clearly more at a low body fat percentage, and that is not a problem.
For a lean bulk or cut, a target weight is therefore less helpful than a target body fat percentage or waist measurement. Use the range as a rough guide and steer by trend weight, waist and body fat. That way you end up at a weight that suits your build.