The birth control pill is a method that uses oral tablets to prevent pregnancy. The pills contain hormones that mimic the body’s natural hormone release. Without birth control pills, estrogen and progesterone are released from your ovaries. When you take birth control pills your body knows that estrogen and progesterone are coming from somewhere else (from 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 prevent pregnancy.
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 and may release an egg, which can cause women to get pregnant while on birth control pills. Missing pills can also cause irregular periods.