Demand generation is like asking your mommy for a toy you really want. You have to tell her about it and ask for it before she can buy it for you.
In business, companies have to tell people about their products or services and convince them they need/want it, so they will buy it. This is called demand generation.
Companies do things like advertising, email campaigns, social media, and events to get people interested in what they are selling. This makes people aware of the product or service and creates demand for it.
Similar to how you have to tell your mommy you really want that toy, companies have to tell people about what they are offering to create a demand for it.