A greenfield project is like making a brand new playground in an open field. Just like how your parents or teachers decide to build a playground from scratch, greenfield projects are also new things that people create from zero. For example, a company might want to make a new factory for making ice cream, but they don't have any land, buildings or machines to use yet. In that case, they will either buy or lease land, then build a new factory and install the equipment needed for making ice cream.
Since everything is new and there isn't anything there before, the company has to decide on every detail, like where to put the machines, how to design the factory, and which recipes to use for the ice cream. They also have to hire and train workers to operate the machines and make ice cream products.
Just like how a new playground is exciting for kids to play on for the first time, a greenfield project is exciting for companies because they get to create everything from scratch and make it exactly how they want it. However, it can also be very challenging and expensive because they have to decide on everything and build everything from scratch.