In the last years of Eastern Han Dynasty of China, Cao Cao went to fight Zhang Xiu. It was in hot summer and the sun sent out huge quantity of heat. They were marching on barren areas and there was not a trace of human habitation and no water source of water. No matter how they try, the generals and soldiers can’t find water. With each mile they passed, several soldiers fell down because of heat stroke. Even the strongest one can’t stand any more. Cao Cao was very anxious. He climbed on to the small hill and stared into distance and found nothing but endless cracked land, turning back, he can only see weak thirsty soldier. It was obvious that they can’t walk too long distance any more.
Cao was a very smart man. He knew if they can’t find water and stay in the barren area for too long time, they will not only miss the opportunity to win a battle, but also loss lots of troops. What is needed is to motivate his men to get out of the barren area. He got a good idea and shouted to his men: hurry up, there is a woods of plum trees with big and sweet plums. We hold on for a little while, then we can eat them.
Upon hearing the words, the soldiers think of the sour plum and their mouth got some saliva, and they cheered up became in high spirit. So they got up the strength and moved on. At last they walked out of the barren area and find water source.
This idiom means in front of troubles, you need to motivate yourself, thereby have more courage to overcome the difficulties and succeed in the end.