Horses should always have access to white salt, which they will consume as they need. White salt is sodium chloride, which is used in very large amounts by the body.
Trace mineral (red or brown mineral blocks or loose minerals) should be provided as well, per your vet’s recommendations, but in general the need for trace minerals is much less.
Growing foals need access to more trace mineral than adults. By separating sodium chloride from trace minerals, horses are allowed to more specifically consume what they need. Foals tend to consume more trace mineral when it is offered loose rather than in block form. Horses that eat more white salt will consume more water, which causes them to produce more urine.