The horse has survived the centuries by running from danger. The three-legged ones didn't make it. So lameness is a serious problem. One cause of lameness is founder or laminitis. The two terms are used interchangeably, although, to me they have different definitions.
Laminitis means inflammation of the sensitive lamina of the foot.
This is the tissue that connects the coffin bone to the hoof wall and sole. Founder implies some incident that can lead to laminitis, but also may involve the entire body in some cases. Eating too much grain, drinking too much water when hot, and allergies to various feed or drugs are examples of incidents, which can cause founder. These things can cause a horse to be generally sick as well as causing laminitis. There may be no one in the world that totally understands what happens to the foot when a horse founders.