The dromedary, or Arabian camel, has one hump and is well suited to desert life. Many people believe that the hump stores water, but it actually stores fat. This fat is an energy reserve, so a camel can travel for days with little food. Camels can also go a long time without drinking, and when they find water they can drink a very large amount in a few minutes. Their wide feet spread their weight on soft sand, and their long eyelashes and closable nostrils protect them from blowing dust. For centuries, desert peoples used camels to carry goods and people between distant towns, and caravans crossed the Arabian Peninsula in this way. Camels also provide milk and wool. Today, trucks do much of the transport, but camels are still loved in the region, and races are held with great excitement.

Question: The word 'reserve' in the passage means