Reflux is a condition which is believed to affect approximately 60% - 70% of babies at various stages of their early lives, to various degrees. It normally starts from birth, or soon thereafter, peaks around 4-6 months and then in most cases disappears by 1 year.
Reflux occurs when the lower oesophagus sphincter (or muscle that controls the movement of substances in and out of the stomach) is underdeveloped; or the baby reacts to something in her diet, causing the contents of the stomach, including the stomach acid, to flow back up the oesophagus (throat). This may be quick and cause no symptoms at all, or may be more prolonged and frequent burning the baby's oesophagus and causing significant discomfort and pain.
In other words the milk your baby has just drunk, cannot be stored securely in the stomach and is pushed back up the oesophagus and vomited out of the baby's mouth, sometimes with great force. As the milk has now been mixed with stomach acid, it burns the baby's oesophagus and throat on its way up.