Sodium

The main salt in the fluid outside your cells, and what sets how much water your body holds.

It is really a water measurement: sodium moves when the balance between salt and water shifts, which is almost always the water.

The range is narrow because the body defends it hard, so genuine values a long way outside it are unusual and matter. Very high triglycerides or protein can make it read falsely low.

Worth discussing any value clearly outside the range, especially with confusion or headaches.