A regimen is a systematic plan for some kind of therapy. If your doctor tells you to eat more vegetables, cut out potato chips, exercise, and take vitamins, she is prescribing a regimen for better health.
It's easy to confuse regimen with regime. While both words can refer to an organized plan for diet, exercise, or medical therapy, only regime can refer to a system of government. If you wash your face twice a day, apply medicine to your acne, and use a special lotion, that's your skin-care regimen. But if a country votes out the ruling party in favor of new leaders, that's a new regime.