It is different from fatigue , which is a loss of energy. Weakness can be used to describe a mental and physical state in which someone doesn't have the muscle strength, for example, to walk. It is common and sometimes difficult to evaluate.
Weakness has many causes.
Weakness itself may be a symptom. Sometimes, no cause is found. Prevention depends on the cause. For example, weakness due to lack of exercise can be prevented with regular exercise. Weakness due to alcohol or diabetic neuropathy can be prevented by not drinking alcohol or controlling diabetes with a proper diet and medications.