So, you know how sometimes you really, really want a toy or a snack or something, but then when you have it, you don't want it as much anymore? That's kinda like demand risk.
It's when people want something a lot at first, but then their interest or need for it changes. This is a problem for businesses that make and sell things. They have to predict how many people will want their product and for how long, so they can make enough to sell but not too much that they have lots of extra stuff left over.
If people suddenly stop wanting something, the business might have lots of leftover things that they can't sell anymore. This can cost them a lot of money and make it harder for them to keep making and selling things. So, businesses have to take demand risk seriously and try to predict it as best as they can.