Your food pipe (called the oesophagus) is a tube that runs from your mouth to your stomach.
Your stomach is where your food is digested.
There is a ring of muscle between your food pipe and your stomach.
When you eat, food travels down your food pipe. The ring of muscle between the food pipe and your stomach opens to let the food go into the stomach.
The ring of muscle then closes again – so that food or acid from your stomach cannot go back up into your food pipe.
Sometimes this ring of muscle is not able to close properly. Some food and acid from your stomach can then flow back up into your food pipe towards your throat.
This causes heartburn – a painful burning feeling in your chest.
If you have heartburn every now and then, this isn’t dangerous.
If you keep getting a lot of heartburn, it is called gastro-oesophageal reflux disease (GORD).
This can cause more serious problems.