Food goes off because bacteria and fungi feed on it. These microorganisms release enzymes that break down the food, which is decomposition. Food preservation slows or stops decomposition by making conditions unsuitable for the microorganisms.
Temperature: enzymes work fastest at a warm optimum temperature, so microorganisms grow quickly in a warm kitchen. In a fridge the low temperature slows their growth and the rate of decomposition. In a freezer it is so cold that they almost stop growing, although they are not killed. Heating food to a high temperature, as in canning, denatures their enzymes and kills them.
Water content: microorganisms need water to grow. Drying removes it. Adding a lot of salt or sugar, as in salted fish or jam, draws water out of the microorganisms by osmosis, so they cannot grow and food lasts longer.
Oxygen: many bacteria and fungi need oxygen for aerobic respiration. Vacuum packing and sealed cans remove the oxygen, so aerobic microorganisms cannot grow and decomposition is slower.