A commodity is something that people buy and sell, like toys or food. In Marxism, a commodity is something that has both use value and exchange value. Use value means that the thing can be used to satisfy a need or a want, like when you eat food when you are hungry. Exchange value means that the thing can be traded for something else, like when you buy a toy with money.
Marxism is a way of understanding how society works. Marx believed that the most important thing in society is work or labor. He thought that the people who work, like the farmers and the factory workers, are the most important people in society. Marx believed that capitalism, the economic system we live in, relies on workers to make things which are then sold as commodities. He said that the problem with this system is that the people who own the factories, the capitalists, make more money than the workers who actually do the work.
Marx believed that we could change society so that workers would own and control the means of production, or the factories and land that are used to produce commodities. He thought that this would be better because workers would all share in the profits, and they would have more control over their lives.
So, when we talk about commodities in Marxism, we are talking about the things that workers produce, and the way that they are bought and sold. Marx believed that if we change the way that we produce and distribute commodities, we can create a more just and equal society.