In the Spring and Autumn Period, Duke Huan of Qi led an army to attack a small state in the north. They went in spring when green grass covered the ground. But when they came back it was winter. Everywhere was white with snow and the wind was howling. The troops lost their way. While everybody was worrying, Guan Zhong, the duke's chief minister, suggested: 'An old horse may know the way.' So the duke ordered several old horse to be selected to lead the army. Finally, they found the way back home.
This idiom refers to the value of experience.