Market demand is like the number of kids that want a toy. If lots of kids want the toy, that means there is a high demand for it. If not many kids want it, then there is a low demand.
Market demand is the amount of things that everybody wants to buy at a certain time. This can be anything, like cars or clothes or food. The more people that want something, the higher the market demand is.
So let's say you have a candy store. If lots of people come into your store wanting candy, that means there is a high market demand for candy. That tells you that you should order more candy from your suppliers, because you're going to sell a lot of it. But if nobody is coming into your store wanting candy, then there is a low market demand for candy, and you should probably order less from your suppliers so that you don't waste your money on candy that won't sell.
So, in short, market demand is like the popularity of something. The more people that want it, the higher the market demand is. It's important for businesses to pay attention to market demand so they can make sure they are selling things that people actually want and not wasting their money on things that nobody is interested in buying.