Our feet are in dark and damp environments most of the time (because we wear shoes which prevent sweat from evaporating). These conditions are perfect for the growth of bacteria which feed on the dead skin cells and oil on our skin. The smell comes from the compounds that these bacteria produce as they grow.
Some of the compounds that those bacteria produce are really really smelly! A really smelly chemical is one that our noses can detect at really low concentrations so there doesn’t need to be much of it before you smell it. Very smelly chemicals often have sulfur (like the smell of skunks or garlic, or the compound they put into household gas supplies) or nitrogen (rotting fish, the smell of poo) in them.