“The pill” is a tablet that contains hormones that mimic the body’s natural release of hormones. Without “the pill”, estrogen and progesterone are released from your ovaries. In some people, their release of these hormones causes heavy periods, severe menstrual cramping, and increased acne. When you are on birth control pills, your body knows that estrogen and progesterone are coming from somewhere else (the pills). Because your ovaries don't need to release hormones, they "take a little nap." While they're "napping," they don't release hormones, and don't release an egg, which is how they work to lighten your period, reduce the menstrual cramps and decrease acne development.
Forgetting pills or taking them at different times of the day can cause the ovaries to "wake up." When that happens, the ovaries release hormones which can cause the heavy periods, severe cramps and acne to return.