I don't really like the heading of this article! It should be "This is how heavy your weights should be if you're trying to gain strength and build muscle". 

Strength training is a key part of fitness that shouldn't be ignored, especially when it comes to weight loss. But just because you know you should be doing it doesn't mean you know exactly how to do it yet.

"If you lift too light, your muscle fibers aren't undergoing enough stress to create physiological and metabolic change in your body," Ridge told POPSUGAR. "If you're lifting too heavy (meaning only getting two to three reps in with mediocre form and only working out for six months or less), your muscles have a very high risk of spraining, tearing, and bruising. Also, your tendons and ligaments will suffer from injury due to premature loading."
I might add that performing only two to three reps is likely not to recruit enough muscle fibers ("inroad") and also not lead to enough stress to create physiological and metabolic change to build muscle and increase strength.

The Americal College of Sports Medicine (ACSM) position stand states that 80% of 1-repetition-maximum (the weight that can only be lifted once successfully) is optimum to produce progression of strengthening. This weight needs to be lifted to muscle failure, or the point at which you can no longer lift the weight (with good form), and at 80% of 1-repetition-maximum, this would be in the 8-15 repetition range for most muscle groups.

• Perform one set of an exercise for each major muscle groups

• Lift the appropriate weight, with perfect form, to the point of not being able to lift it

• Repeat this at least once per week

See article in POPSUGAR.Fitness