SodiumNa
The main salt in blood, and the number that tracks your body's water balance.
Sodium is the most abundant electrolyte outside your cells. The body holds its concentration inside a narrow band, because the amount of sodium relative to water is what determines how water moves between blood and cells.
It matters because the blood sodium level is really a statement about water, not salt intake. It falls when there is too much water for the sodium present, which is why endurance athletes who drink heavily during long events can push it down, and it rises with dehydration.
In practice it is read alongside potassium, chloride and bicarbonate as a group, and interpreted with hydration and any medication in mind rather than as a comment on how much salt someone eats.