Diarrhoea usually means doing a poo more often than normal for you, and that the poo itself is more watery than normal. You can get it on its own or with vomiting.
The most common cause is 'gastroenteritis' which means inflammation of your gut. This is usually caused by a viruses which you catch from other people without meaning to. Sometimes it's from bacteria in dodgy food which hasn't been cooked properly. There are lots of more rare causes but these are only worth thinking about if your diarrhoea lasts more than a few days, or you have been abroad somewhere exotic recently.