Food was cooked, quickly frozen, and then dehydrated in a special vacuum chamber. Freeze-dried food didn't need to be refrigerated and would last a long time. To make most freeze-dried foods, astronauts squeeze water into the food packages and then eat the food after it absorbs the water.
The astronauts had to eat dried food because they didn't know when their food was going to spoil or when they would get more. Its like living of a weeks worth of groceries for a few months. It took the station a long time to send food because of all the light years ( how many years it takes light to go through space) it takes to get to the space ship with hungry astronauts.
