"No longer has enough fluid to work properly." Here "fluid" means

Dehydration happens when the body loses more water than it takes in, and no longer has enough fluid to work properly.

There are many ways to lose too much water. Heavy sweating, fever, vomiting and diarrhoea can all do it, and so can simply not drinking enough.

Some people are more at risk than others. Babies and small children are more likely to have diarrhoea or vomiting. Some older people lose their sense of thirst, so they do not drink enough. People who exercise or work outdoors in hot weather sweat a great deal, and people with certain illnesses may lose water faster than usual.

The signs in adults include feeling very thirsty, a dry mouth, sweating and passing urine less than usual, dark-coloured urine, dry skin, tiredness and dizziness.

For most people, the answer is simple: drink enough water or other fluids through the day, and more when it is hot, when you are active, or when you are ill.