Salt is mostly sodium chloride.
Sodium is one of the electrolytes that the body tries to keep in balance.
Too much salt intake means the body has to work harder to keep the balance which in turn puts a strain on the heart, kidneys and other body systems. Raised blood pressure is associated with cardio vascular disease
Most processed foods ie tinned, ready meals, snack foods use salt to make them more tasty and as a preservative so it's probable that some people eat over the recommended guidelines without realising.