An engine needs each of its part to work. It has gears which have wheels. Each wheel has cogs, or tiny teeth that fit together, making the wheel turn, the engine run. Every cog is essential to that engine.
People can also be cogs — they are the workers who, day in and day out, perform their duties seemingly with no end in sight. Feeling disheartened, they might say, "I'm just a cog in the system." If you hear this, remind them of the true definition of the term cog — an instrumental part of the whole.