Ok kiddo, so imagine you have a toy that you really like and you don't want to share it with anyone. But someone else wants to play with your toy and they offer you some money if you let them play with it for a little while. This might seem like a good deal, but it's not really fair to you because your toy is worth more than the money they're offering. That's kinda like what greenmail is.
You see, sometimes a person or a company will buy up a bunch of shares in another company. Then, they might threaten to take over the company unless the owners of the company pay them a lot of money. This is called greenmail.
It's like the person who wants to play with your toy is the company that buys a bunch of shares and the toy is the company they want to take over. The owners of the company might feel like they don't have a choice but to pay the greenmailer the money they're asking for, even though the company is worth more than that.
So, greenmail is kind of like a bully who tries to make people give them money they don't really deserve. It's not a nice thing to do, and it's not fair to the people who own the company.